Description
We currently support platform.python_implementation and platform_python_implementation. setuptools' previous fork of packaging did support python_implementation until it was replaced by platform_python_implementation in pypa/setuptools@3bd5118#diff-0c04a5cfcbd2a4bdd1121db108b79d3d.
python_implementation existed in setuptools until that commit and the death of MarkerEvaluation, see https://github.com/pypa/setuptools/blob/f8b1293c408bbb652bec3f2ae6e5b4f33f3ca55e/pkg_resources/__init__.py#L1389 for the full (?) list of what was supported there. It also briefly existed in PEP 508 until February (https://hg.python.org/peps/rev/655a101719a5 replaced it with platform_python_implementation).
The one thing we're missing from what setuptools historically supported is python_implementation; we have everything else. This adds more duplication, but its removal broke some packages (e.g., some versions of html5lib).