TST: possible improvements in tox integration

[neutrinoceros]

We are currently using https://github.com/ymyzk/tox-gh-actions

There's a new player in town: https://github.com/tox-dev/tox-gh
This one is part of the tox organisation, so I expect it'll probably outlive tox-gh-actions. I'll attempt a migration and see if it's already mature enough for our needs.

Furthermore, and as I noted in #426 setup-python now supports setting multiple python versions in a single job
actions/setup-python@main/docs/advanced-usage.md#specifying-multiple-pythonpypy-version
In combination with #426, it might be possible to save a lot on build time. The question is how to integrate it with tox in CI.