Add intermediate is_cpython variable.
Add intermediate is_cpython variable.

pfmoore/pip,rouge8/pip,pypa/pip,xavfernandez/pip,rouge8/pip,sbidoul/pip,sbidoul/pip,rouge8/pip,pradyunsg/pip,pradyunsg/pip,pfmoore/pip,pypa/pip,xavfernandez/pip,xavfernandez/pip