Summary

• use packaging.version.parse in the Git test instead of the removed pkg_resources.parse_version
• rely on the existing packaging>=20 tox dependency

Fixes #414

Validation

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• Not run locally

Remote CI

• Tox run 26342344094 completed with bootstrap and build passing.
• The test* jobs failed in test/git/test_invocations.py around setup.py develop / isolated pip installs (--find-links handling and missing setuptools>=40.8.0 in local links).
• Recent master Tox runs 26009541783 and 26010403410 are also failing on the same workflow, so this does not appear to be caused by the one-line import replacement in this PR.