Attempting to build a cython extension to a python package, not creating shared object (.so) file

I have attempted to use the answer here to add the building of a cython extension into my package. It currently cythonizes the code to produce a .c file from the .pyx file but doesn't create a shared object .so file, as such when I try to import the package, and one of the modules attempts to import the shared object file it cannot find it.

My setup.py file (this is slightly cut-down) is like so:

from setuptools import setup
from setuptools.extension import Extension
import os
import numpy
from Cython.Build import cythonize

mypackage_root_dir = os.path.dirname(__file__)
with open(os.path.join(mypackage_root_dir, 'requirements.txt')) as requirements_file:
    requirements = requirements_file.read().splitlines()

extensions = [Extension(
    name="package.submodule.foo",
    sources=["package/submodule/foo.pyx"],
    include_dirs=[numpy.get_include()],
    )
]

setup(name='package',
      version=0.1,
      description='...',
      author='my name',
      author_email='my email',
      url="...",

      include_package_data=True,
      packages=['package',
                'package.submodule1',
                'package.submodule2',
                'package.submodule', # the one that uses the pyx file
      ],
      ext_modules = cythonize(extensions),
      install_requires=requirements,
)

How can I fix this such that I can get the setup.py file to build the shared object file when python setup.py install is run?