Backwards incompatible breakage in marker parsing

[Geoffrey Sneddon]

See <https://gsnedders.github.io/python-marker-test/results.html> for a summary of what versions of setuptools support what environment markers.

Wily shipped with 18.4 and that Xenial's 20.7.0 doesn't support "python_implementation" unlike 18.4, although later releases of setuptools fixed this backwards incompatible change. It would be nice, for the sake of compatibility, if the Ubuntu package could patch this backwards incompatible change.

packaging fixed this upstream with the ALIASES in <https://github.com/pypa/packaging/blob/16.7/packaging/markers.py#L64>, for reference.

An easy test for this is pkg_resources.evaluate_marker("python_implementation == 'CPython'") — this should not throw an exception (and should return True on CPython, but False otherwise).

[Geoffrey Sneddon (geoffers)]

https://github.com/pypa/packaging/commit/3f22589b6888c5a600ae5cf71730c4b0e1c44913 is probably the minimal change needed to fix this. Upstream then had https://github.com/pypa/packaging/commit/775fb67833d00179286dce8ce673b845d5cecdc0 which technically does change behaviour in another edge-case.