MAINT: some TLC for old versions

[bsipocz]

We run into a couple of easy to fix issues while investigating #3584

I still have issues directly running sphinx-build in some old python env, but all those python versions do work for me™️ with tox -e build_docs, so I stopped investigating further.

(typical flavour of the failure I see is automodapi related with something like this:

Handler <function process_automodsumm_generation at 0x123f96f80> for event 'builder-inited' threw an exception (exception: No module named 'astroquery.esa.integral')

I would say it probably wouldn't hurt to find and declare minimum sphinx and sphinx extension versions, too.

[bsipocz]

Just for cross references, we may need to look into astropy/sphinx-automodapi#119 and the workaround used by spacetelescope/roman_datamodels#156?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.22%. Comparing base (a1fee2a) to head (47f2065).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3585   +/-   ##
=======================================
  Coverage   73.22%   73.22%           
=======================================
  Files         226      226           
  Lines       21000    21000           
=======================================
  Hits        15378    15378           
  Misses       5622     5622           

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