Configuring for pure-python

[mgedmin]

• Re-apply the pure-python template.
• Edit .meta.toml to use testenv-setenv instead of testenv-additional (because without Avoid generating an empty setenv = meta#281 this breaks tox)
• Wrap a comment in docs/conf.py (without which tox -e lint fails with a flake8 error in docs/conf.py)
• Drop Python 3.7 and add Python 3.12 in setup.py, mention that in CHANGES.rst

This fixes tox -e docs as a side effect.

Other changes were performed by the config-package.py script. I'll miss some of those prettily-aligned tables in the test data, but c'est la vie.

Closes #76.

[mgedmin]

I should've said yes, when config-package.py asked me if it should modify branch protection rules, eh?

[mgedmin]

I should've said yes, when config-package.py asked me if it should modify branch protection rules, eh?

I've done so now.