https://tox.wiki/en/latest/faq.html#testing-end-of-life-python-versions is very helpful, except where tox now creates a .tox/.pkg virtualenv using Python 3.12 to run pyproject_api to run setuptools.build_meta, and this venv ends up with setuptools version 65.6.3, which doesn't support Python 3.12.
I think setuptoos 65.6.3 is the one bundled with virtualenv 20.22?
I don't know how to ask tox to please pip install -U setuptools inside the .pkg virtualenv before it tries to run pyproject_api. Or how to use a different Python for .pkg.