[Distutils] Wheels and Python implementation markers

[Alex Grönholm]

What is the "correct" way to specify (for building wheels) in setup.py 
that a dependency should only be installed on CPython? If so, how should 
I go about it? Nothing I've tried seems to work right. The 
python_implementation marker worked up until packaging 16.5 and then 
platform_python_implementation does not work with older versions of pip.