Fix doc generation and PyPI uploads by pinning to Python 3.7

[maxbelanger]

• Per the docs, ReadTheDocs now requires a version 2 config file. In it, we pin the version used to generate the docs to Python 3.7, which matches what is currently tested (and green) in CI.
• PyPI uploads were still configured to run on the latest version of Python (currently 3.12). We pin this back to Python 3.7, which make it less likely to break in mysterious ways.

[maxbelanger]

This now matches what we actually run to test that we can generate docs in the CI workflow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.26%. Comparing base (3173f6e) to head (0113034).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #500   +/-   ##
=======================================
  Coverage   64.26%   64.26%           
=======================================
  Files          31       31           
  Lines       53579    53579           
  Branches     3905     3905           
=======================================
  Hits        34430    34430           
  Misses      18989    18989           
  Partials      160      160           

Flag	Coverage Δ
integration	64.17% <ø> (ø)
unit	63.70% <ø> (ø)

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