Only use _cpython_tags for CPython
Only use _cpython_tags for CPython

Since the behavior for both of these functions is the same, there is no
behavior change here. This will let us simplify `_cpython_tags` a bit.

xavfernandez/pip,xavfernandez/pip,pypa/pip,sbidoul/pip,pradyunsg/pip,sbidoul/pip,xavfernandez/pip,pfmoore/pip,pradyunsg/pip,pypa/pip,pfmoore/pip