Sphinx v9 support

[2bndy5]

• Closes Compatibility with Sphinx 9 #477 (actually cherry picked changes from that branch to preserve git attribution)
• resolves Sphinx v9.0 breaks theme #475

Putting this patch together felt like... If this project was an old wooden ship, then every new version of sphinx (since v6) has been a multitude of new leaks in the hull. This branch is comprised of band-aids to keep the ship afloat.

I got the CI passing, but there's serious changes in sphinx v9 that need to be addressed in the future.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.18%. Comparing base (d5220aa) to head (19c34a4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #484   +/-   ##
=======================================
  Coverage   82.18%   82.18%           
=======================================
  Files          65       65           
  Lines        8660     8660           
=======================================
  Hits         7117     7117           
  Misses       1543     1543           

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