pip3 install error: (ValueError: source code string cannot contain null bytes)

I am trying to install the package using pip3, for example, the following command:

sudo pip3 install testresources setuptools==49.6.0 

But, get the following error:

Traceback (most recent call last):
  File "/usr/bin/pip3", line 9, in <module>
    from pip import main
ValueError: source code string cannot contain null bytes

What I have tried:

Remove pip3 by purge and install it again by sudo apt-get install python3-pip

If it helps, the python version is 3.6.9.