Test more python versions

We currently only test whatever version of python happens to be in our CI containers; currently the relatively old python 3.9 on debian bullseye.

We ought to test on at least a relatively new python as well. We could e.g. use tox for this, or just run our tests in multiple containers. The newest stable debian container (bookworm) only has 3.11 though, and it'd be useful to test on at least 3.12 (see e.g. #40019 (closed))