Python 3.12 compatibility; docker, venv/virtualenv

[ix5]

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)
  

What changes does this Pull Request introduce?

Compatibility for Python 3.12 and a few stray nits

• Mentions needed virtualenv package in docs, make pip and virtualenv related commands more consistent
• Require setuptools in setup.py (mostly because of pkg_resources)
• docker: Use buildkit in another instance of the Makefile, remove version from compose file, improve Dockerfile formatting
• Mark Python 3.12 support in setup.py and use it in Dockerfile, GitHub Actions

Why is this necessary?

Python 3.12 changes: What's new

gh-95299: Do not pre-install setuptools in virtual environments created with venv. This means that distutils, setuptools, pkg_resources, and easy_install will no longer available by default; to access these run pip install setuptools in the activated virtual environment.

• We were using python -m venv and virtualenv interchangeably in the docs
• In Python 3.12, python -m venv seemingly no longer pulls in pip and setuptools, so switch to virtualenv instead
• python setup.py develop is now even more heavily discouraged

[jelmer]

FWIW these changes LGTM, apart from the merge conflicts :)

[ix5]

Might also be time to move on from python setup.py {sdist, bdist} and use the "official" PyPA build package instead, although that's another dependency. PyPA recommends it (via stackoverflow)