Finish transition from tox to Nox for current continuous integration checks and weekly tests

[namurphy]

This PR concludes the switchover from tox to Nox. I wanted to finish this ahead of the PlasmaPy Summer School so that participants won't need to learn how to work with tox (or more specifically, tox.ini).

The changes in this PR include:

• 📚 I added a Nox session to build docs against the development versions of Sphinx, sphinx_rtd_theme, and nbsphinx.
• 🧪 I added a Nox session to run tests against the development versions of NumPy, Astropy, xarray, lmfit (since we had a breaking change come up recently), and pandas.
• 🧪 I switched over weekly tests to entirely using Nox sessions
• ♻️ I consolidated the separate workflows for building docs and running tests in CI to a single workflow.
• ✂️ I deleted tox.ini in its entirely, and there was much rejoicing. Using tox has overall been great, except for the INI-formatted configuration files! (We still have mypy.ini...but I'm hoping that's only temporary.)
• ✂️ I removed requirements.txt, as inspired by Requirements files need to be rethought #2647. It's not being used in CI anymore or by any Nox sessions, so we don't need it. Plus, IDEs are much more adept at getting requirements information from pyproject.toml than they used to be.
• ✂️ I removed the requirements files used only by tox environments. These have been superseded by the requirements files used by Nox sessions.
• ✨ I made it so that weekly tests can be run for PRs that are labeled with "run weekly tests in CI".

Some notes:

• There were a few checks that I wasn't able to get working.
  • 🧪 Testing against the lowest-direct versions of dependencies for Python 3.11 & 3.12.
  • 🧪 Testing against the development version of h5py.
  • 🌠 Building the docs on Windows or macOS, since the sudo apt install step for pandoc and graphviz is specific to Ubuntu.
  • 🦺 I added mypy to the tests dependency set, primarily because then we won't have requirements information defined in noxfile.py that we have to update.

Remaining tasks

• Clean up noxfile.py
• Add more changelog entries
• Possibly spin off some of the smaller, more isolated changes into distinct PRs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.15%. Comparing base (b0cecc8) to head (e456926).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2694      +/-   ##
==========================================
+ Coverage   94.36%   95.15%   +0.79%     
==========================================
  Files         107      107              
  Lines        9458     9458              
  Branches     2184     2184              
==========================================
+ Hits         8925     9000      +75     
+ Misses        345      276      -69     
+ Partials      188      182       -6     

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[namurphy]

📝 This is saying that we should run tests...

• If we manually trigger it (via workflow dispatch)
• It's a pull request that has been labeled with "run weekly tests in CI"
• As a cron job if it's on the main branch of PlasmaPy/PlasmaPy.

[sapols]

I’m probably not the best person to review this since I wasn’t familiar with nox before this PR, but the motivation to move to it is clear and well documented. I see the benefits over tox so I support the move, and it looks to me like you did everything correctly here.

[sapols]

How big of a deal is the one failing weekly tests check?

[namurphy]

@sapols — Thank you kindly for the reviews! 🌱 It's much appreciated!

How big of a deal is the one failing weekly tests check?

We'll need to address the astropy-dev error before our next release, so I'll create a separate issue to look into it. The error is already occurring in our weekly tests on main, so it's unrelated to this PR.