TST: test on GHA (⏰ wait for #238)

[neutrinoceros]

start migrating tests to GitHub Actions
So far I couldn't get data-requiring tests to not skip, but I think this is symptom of a bigger issue with the requires_ds decorator that may not work at all with pytest.
I'll try to make this more useful before I open for review

[neutrinoceros]

I finally reported the issue with requires_ds upstream: yt-project/yt#4692

[neutrinoceros]

After diving in I realised that requires_ds is in fact pretty deeply rooted into yt's answer testing framework, which is not pytest compatible. However, I also reckon that yt_astro's tests seem only written around that framework out of convenience: the "answer objects" that are produced are very small (single floats, or very small numpy arrays), so it should be relatively easy to rewrite them and get rid of nose. I'll try to have a go at it tomorrow. Meanwhile, let's see if it's possible to add Python 3.12 to the test matrix here and get rid of #232

[neutrinoceros]

What's not figured out yet is how to run mpi tests: I migrated them to pytest but I couldn't actually run them yet (maybe I'm hitting a deadlock).

[neutrinoceros]

I guess splitting this work over two pull requests (migration to pytest then migration to GHA) would help with making sure I'm not breaking anything.