Convert return values in pep425tags.get_supported
Convert return values in pep425tags.get_supported

Now we can incrementally use utility functions from pep425tags without
switching everything at once or converting in multiple places.

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