[Bug 1589607] Re: Backwards incompatible breakage in marker parsing

[Geoffrey Sneddon]

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.

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Title:
  Backwards incompatible breakage in marker parsing

Status in python-setuptools package in Ubuntu:
  New

Bug description:
  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).

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