Don't install setup_requires when run as a PEP-517 backend.

Update packaging to 20.4

docs: Suggestion - Provide consistency in the description of setup.cfg

Improve safe_version documentation

Change load_module to exec_module

Remove superfluous 'v' in CHANGES.rst

This was an artifact of the fact that I did not use `tox -e finalize`
to do the latest release in order to make a patch release.

Fix issue with distutils warning

Prefer included distutils even without importing setuptools. Closes #2259.

Add tests capturing expected distutils behavior.

Bypass pytest-virtualenv due to bugs in 'run' command. Instead, use jaraco.envs.VirtualEnv and build a bespoke fixture with a run command.

Under PEP-517, installing build dependencies is up to the frontend.

Closes gh-2303

Set up GHA to test on Python 3.9-beta and 3.8-dev