I got nailed by this same error this morning; after 4 hours of trying a tonne of things, and using up my full swearbox donations for the month, I stumbled on a solution.

I have Python3 installed via homebrew, but Xcode 12 updated over the weekend and hijacked it on my machine. Whatever the Xcode install of Python 3 included, it also tries to set CFLAGS that contain the ARM architecture - and that's causing the error.

If you run python3 -m sysconfig it will output a huge list of variables. Have a scan of them, and if it's pointing at your XCode install of python instead of one in your /usr/local/{wherever brew installed version 3.x of python} then it might be the same issue.

I eventually fixed this by:

• deactivating my virtualenv for this project
• deleting the virtualenv folder for the project
• ran brew doctor and followed advice to make sure the right folders & links are setup
• making sure export PATH=/usr/local/bin:/usr/local/sbin:~/bin:$PATH is at the start of my .zshrc (or if you're still using bash, .bash_profile) path updates
• starting a new terminal window
• checking the python I'm pointing at is the brew one with which python3
• switching to the project and running python3 -m venv ~/.virtualenvs/project-name
• activating that virtualenv with source ~/.virtualenvs/project-name/bin/activate
• then finally pip3 install -r requirements.txt

Hopefully this will work for you (or someone), then I know those 4 hours were worth it.

$ pip3 install mysqlclient
Looking in indexes: http://mirrors.ugtt.edu/pypi/simple
Collecting mysqlclient
  Downloading http://mirrors.ugtt.edu/pypi/packages/3c/df/59cd2fa5e48d0804d213bdcb1acb4d08c403b61c7ff7ed4dd4a6a2deb3f7/mysqlclient-2.0.3.tar.gz (88kB)
     |████████████████████████████████| 92kB 1.1MB/s
Installing collected packages: mysqlclient
  Running setup.py install for mysqlclient ... error
    ERROR: Command errored out with exit status 1:
     command: /Users/e123/Projects/python/envs/pi2/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-install-mkpiwyap/mysqlclient/setup.py'"'"'; __file__='"'"'/private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-install-mkpiwyap/mysqlclient/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-record-fhoi4_w2/install-record.txt --single-version-externally-managed --compile --install-headers /Users/e123/Projects/python/envs/pi2/include/site/python3.8/mysqlclient
         cwd: /private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-install-mkpiwyap/mysqlclient/
    Complete output (132 lines):
    mysql_config --version
    ['8.0.22']
    mysql_config --libs
    ['-L/usr/local/opt/mysql-client/lib', '-lmysqlclient', '-lssl', '-lcrypto', '-lresolv']
    mysql_config --cflags
    ['-I/usr/local/opt/mysql-client/include/mysql']
    ext_options:
      library_dirs: ['/usr/local/opt/mysql-client/lib']
      libraries: ['mysqlclient', 'resolv']
      extra_compile_args: ['-std=c99']
      extra_link_args: []
      include_dirs: ['/usr/local/opt/mysql-client/include/mysql']
      extra_objects: []
      define_macros: [('version_info', "(2,0,3,'final',0)"), ('__version__', '2.0.3')]
    running install
    running build
    running build_py
    creating build
    creating build/lib.macosx-10.14.6-x86_64-3.8
    creating build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/__init__.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/_exceptions.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/connections.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/converters.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/cursors.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/release.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    copying MySQLdb/times.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb
    creating build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/__init__.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/CLIENT.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/CR.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/ER.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/FIELD_TYPE.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/FLAG.py -> build/lib.macosx-10.14.6-x86_64-3.8/MySQLdb/constants
    running build_ext
    building 'MySQLdb._mysql' extension
    creating build/temp.macosx-10.14.6-x86_64-3.8
    creating build/temp.macosx-10.14.6-x86_64-3.8/MySQLdb
    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -iwithsysroot/System/Library/Frameworks/System.framework/PrivateHeaders -iwithsysroot/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/Headers -arch arm64 -arch x86_64 -I/usr/local/opt/openssl/include -Dversion_info=(2,0,3,'final',0) -D__version__=2.0.3 -I/usr/local/opt/mysql-client/include/mysql -I/Users/e123/Projects/python/envs/pi2/include -I/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/include/python3.8 -c MySQLdb/_mysql.c -o build/temp.macosx-10.14.6-x86_64-3.8/MySQLdb/_mysql.o -std=c99
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:45:
    In file included from /Library/Developer/CommandLineTools/usr/lib/clang/12.0.0/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:32:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/cdefs.h:807:2: error: Unsupported architecture
    #error Unsupported architecture
     ^
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:45:
    In file included from /Library/Developer/CommandLineTools/usr/lib/clang/12.0.0/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:33:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/machine/_types.h:34:2: error: architecture not supported
    #error architecture not supported
     ^
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:45:
    In file included from /Library/Developer/CommandLineTools/usr/lib/clang/12.0.0/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdint.h:52:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:55:9: error: unknown type name '__int64_t'
    typedef __int64_t       __darwin_blkcnt_t;      /* total blocks */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:56:9: error: unknown type name '__int32_t'
    typedef __int32_t       __darwin_blksize_t;     /* preferred block size */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:57:9: error: unknown type name '__int32_t'
    typedef __int32_t       __darwin_dev_t;         /* dev_t */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:60:9: error: unknown type name '__uint32_t'
    typedef __uint32_t      __darwin_gid_t;         /* [???] process and group IDs */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:61:9: error: unknown type name '__uint32_t'
    typedef __uint32_t      __darwin_id_t;          /* [XSI] pid_t, uid_t, or gid_t*/
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:62:9: error: unknown type name '__uint64_t'; did you mean 'uint64_t'?
    typedef __uint64_t      __darwin_ino64_t;       /* [???] Used for 64 bit inodes */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/_types/_uint64_t.h:31:28: note: 'uint64_t' declared here
    typedef unsigned long long uint64_t;
                               ^
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:45:
    In file included from /Library/Developer/CommandLineTools/usr/lib/clang/12.0.0/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdint.h:52:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:68:9: error: unknown type name '__darwin_natural_t'
    typedef __darwin_natural_t __darwin_mach_port_name_t; /* Used by mach */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:70:9: error: unknown type name '__uint16_t'
    typedef __uint16_t      __darwin_mode_t;        /* [???] Some file attributes */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:71:9: error: unknown type name '__int64_t'
    typedef __int64_t       __darwin_off_t;         /* [???] Used for file sizes */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:72:9: error: unknown type name '__int32_t'
    typedef __int32_t       __darwin_pid_t;         /* [???] process and group IDs */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:73:9: error: unknown type name '__uint32_t'
    typedef __uint32_t      __darwin_sigset_t;      /* [???] signal set */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:74:9: error: unknown type name '__int32_t'
    typedef __int32_t       __darwin_suseconds_t;   /* [???] microseconds */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:75:9: error: unknown type name '__uint32_t'
    typedef __uint32_t      __darwin_uid_t;         /* [???] user IDs */
            ^
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types.h:76:9: error: unknown type name '__uint32_t'
    typedef __uint32_t      __darwin_useconds_t;    /* [???] microseconds */
            ^
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:45:
    In file included from /Library/Developer/CommandLineTools/usr/lib/clang/12.0.0/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdint.h:53:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types/_intptr_t.h:30:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/machine/types.h:37:2: error: architecture not supported
    #error architecture not supported
     ^
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:45:
    In file included from /Library/Developer/CommandLineTools/usr/lib/clang/12.0.0/include/stdint.h:52:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdint.h:53:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/_types/_intptr_t.h:32:9: error: unknown type name '__darwin_intptr_t'
    typedef __darwin_intptr_t       intptr_t;
            ^
    In file included from MySQLdb/_mysql.c:29:
    In file included from /usr/local/opt/mysql-client/include/mysql/mysql.h:46:
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/types.h:81:
    /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/machine/endian.h:37:2: error: architecture not supported
    #error architecture not supported
     ^
    fatal error: too many errors emitted, stopping now [-ferror-limit=]
    20 errors generated.
    error: command 'clang' failed with exit status 1
    ----------------------------------------
ERROR: Command errored out with exit status 1: /Users/e123/Projects/python/envs/pi2/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-install-mkpiwyap/mysqlclient/setup.py'"'"'; __file__='"'"'/private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-install-mkpiwyap/mysqlclient/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /private/var/folders/h6/q2hryq8n3hj_9lmqznt05jwm0000gn/T/pip-record-fhoi4_w2/install-record.txt --single-version-externally-managed --compile --install-headers /Users/e123/Projects/python/envs/pi2/include/site/python3.8/mysqlclient Check the logs for full command output.

Other things:

Yesterday morning I upgraded the version of macos from 10.15.4 -> 10.15.7 After that, all python virtual environments have been abolished.

Here is how I fixed this problem：

The above error is that the underlying system platform cannot be identified, so I just specified it separately. I don't know if there are other problems with this installation. So far, everything is ok.

$ export CFLAGS="-D__x86_64__"  // OR export ARCHFLAGS="-arch x86_64"
$ pip3 install mysqlclient
Looking in indexes: http://mirrors.ugtt.edu/pypi/simple
Collecting mysqlclient
  Downloading http://mirrors.ugtt.edu/pypi/packages/3c/df/59cd2fa5e48d0804d213bdcb1acb4d08c403b61c7ff7ed4dd4a6a2deb3f7/mysqlclient-2.0.3.tar.gz (88kB)
     |████████████████████████████████| 92kB 1.2MB/s
Installing collected packages: mysqlclient
  Running setup.py install for mysqlclient ... done
Successfully installed mysqlclient-2.0.3

I had this problem. It looked to me like I was using brews python3 so I was confused.

To fix it I installed python 3.8.3 via pyenv, and created a virtualenv with which python 3.8 and in that venv I could install mysqlclient.

This solved my problem on MacOs catalina:

ARCHFLAGS="-arch x86_64" pip3 install mysqlclient

try using brew there are a lot of posts on failing install on mac

brew install mysql

also the error "#error Unsupported architecture" - it might be that you are using Mojave and the latest mysqlclient was updated for Catalina or try this or their git recommendation for macOS