Drop legacy distutils fallback in setup.py

The command "python setup.py test" hasn't been used for testing since
ea10824e. Can greatly simplify setup.py
and testing by removing the unnecessary workarounds for this.

The distutils package is not recommended for use and unnecessary for
modern Python environments. Use only setuptools instead. From
https://docs.python.org/3/library/distutils.html:

> Most Python users will not want to use this module directly, but
> instead use the cross-version tools maintained by the Python Packaging
> Authority. In particular, setuptools is an enhanced alternative to
> distutils ...
>
> The recommended pip installer runs all setup.py scripts with
> setuptools, even if the script itself only imports distutils. Refer to
> the Python Packaging User Guide for more information.