Modernize & restore testing infrastructure

[cphyc]

Testing infrastructure has been broken since support for Python 3.9 in yt has been dropped.

We should:

• find a way of using nose for testing purposes, despite its lack of support for Python 3.10+, see Restore CI #264. A longer term solution is to migrate fully onto pytest, see TST: migrate from nosetest to pytest #238
• drop support for 3.9
• Migrate away from CircleCI, see TST: test on GHA (⏰ wait for #238) #229
• Update the conda feedstock so that yt-astro-analysis can be installed through conda on Python 3.13, see Rebuild for python 3.13 conda-forge/yt_astro_analysis-feedstock#25.