MAINT/BLD: Python CI support and docs build fixes

[bocklund]

• Add support in CI for Python 3.12
• Drop support for Python 3.7 as it's EOL as of June 2023 https://devguide.python.org/versions/
• Drop a really old pin on furo because we can upgrade now (pin was forcing old sphinx versions to be installed, which is what was breaking our docs builds in CI)
• Fixes for new versions of sphix:
  1. Language is no longer allowed to be None, set to 'en' which is the default
  2. Fix extlinks that requires a %s in the caption (added support for that in v4.0, removed support for deprecated way without the %s in v6.0 Remove deprecated items for Sphinx 6.0 sphinx-doc/sphinx#10471)

[bocklund]

No changes need for our wheel building flags, which are currently including 3.12 and not including 3.7 anyway

      - name: Build wheels
        uses: pypa/cibuildwheel@v2.16.2
        env:
          # PyPy wheels not allowed because SciPy (build requirement) is not available
          CIBW_BUILD: cp3*-*
          CIBW_SKIP: cp35-* cp36-* cp37-* *-musllinux_*

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8fd0f06) 90.47% compared to head (335cada) 90.46%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #508      +/-   ##
===========================================
- Coverage    90.47%   90.46%   -0.01%     
===========================================
  Files           50       50              
  Lines         7871     7868       -3     
===========================================
- Hits          7121     7118       -3     
  Misses         750      750              

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[richardotis]

Coverage analysis is signalling that the importlib_metadata backport can be removed now. I'm fine either way. If it is removed from __init__.py, be sure to update the dependencies accordingly.