CI: revert macos-14 runners, migrate to tox v4-exclusive settings, add Python 3.13

[pajod]

1. A few other PRs have degraded CI results because Github does not provide Python 3.7 on the default macos-latest runners.

   • Suggestion: just drop back to macos-13, an try the macos-latest image just for a single known-good version.
   • Considered alternative: keep defaults and special case the other way around: not great while the macos-14 images also have unrelated issues.

2. The tox.ini contained supersedes settings and a reference to unpleasant behaviour in a scenario we may entirely avoid by using the package / no_package flags instead of skipsdist and usedevelop.

   • Suggestion: Just assert we are using tox v4 and let tox prepare something more like what we actually need for each tests

3. The upcoming Python 3.13 version comes with some interesting changes (see Check that threads works well with Gunicorn and Python 3.13 (I think that It doesn't) #3205) which warrant some expansion in the area of automatic tests

   • Suggestion: Start by adding it to the test matrix, but suppress its (red) return status for now
   • Will by itself not do much as long as tests are always tried with all dependencies, and there is no 3.13 greenlet yet: Fail to build with Python 3.13 python-greenlet/greenlet#392