PIN: Jinja < 3.1

[choldgraf]

This pins Jinja < 3.1, so that we don't run into the bug described in:

• Jinja 3.1 is incompatible with Sphinx 3.5, and breaks Jupyter Book w/ this configuration #1676

closes #1676

[choldgraf]

Will merge this one if the tests are happy, because it'll un-break our failing tests

[codecov]

Codecov Report

Merging #1678 (be2eea8) into master (77961b4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1678   +/-   ##
=======================================
  Coverage   91.30%   91.30%           
=======================================
  Files           7        7           
  Lines         690      690           
=======================================
  Hits          630      630           
  Misses         60       60           

Flag	Coverage Δ
pytests	91.30% <ø> (ø)

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