Fix documentation build by lowering jinja2 version

[JBGreisman]

This is a quick fix to the documentation build. There is a current issue with sphinx due to a newer version of Jinja2 (readthedocs/readthedocs.org#9037 (comment)).

After this fix, I'll look into making a more future-proof documentation build -- might require updating to using a more recent Python version when building documentation.

[JBGreisman]

This fix corrects documentation builds locally -- our current CI doesn't explicitly test the build of the documentation, and it would be a good idea to add such a check in the future.

[codecov-commenter]

Codecov Report

Merging #202 (0719bff) into main (e919c77) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #202   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files          45       45           
  Lines        1803     1803           
=======================================
  Hits         1774     1774           
  Misses         29       29           

Flag	Coverage Δ
unittests	98.39% <ø> (ø)

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