Re-add Python 3.12, include setuptools, require sphinx>=5.02

[jaydenpersonnat]

This PR resolves unit tests failing on Python 3.12. Changes include:

• Including setuptools in requirements and setup.py
• Re-adding Python 3.12 in unit-tests.yml
• Requiring sphinx to be greater than or equal to version 5.0.2, rather than strictly equal.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.47%. Comparing base (c0f6b9a) to head (56fa620).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #151      +/-   ##
===========================================
- Coverage    74.47%   74.47%   -0.01%     
===========================================
  Files           11       11              
  Lines         3785     3784       -1     
===========================================
- Hits          2819     2818       -1     
  Misses         966      966              

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[jaydenpersonnat]

• Updated readthedocs.yaml to use Python 3.9.
• Updated requirements.txt in \docs to require sphinx>=5.0.2

[StokesMIDE]

Looks good!

Do you think the various sphinxcontrib-* packages should have their versions unpinned as well? It doesn't matter too much either way; it can be addressed if/when there's a problem in the future.