revert: Remove Jinja2 restrictions given nbconvert v6.4.5

[matthewfeickert]

Description

Revert PR #1824 given updates in nbconvert v6.4.5 (c.f. spatialaudio/nbsphinx#641 (comment)). The issues described there were resolved upstream of nbsphinx in jupyter/nbconvert#1737.

Resolves #1823

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Revert PR #1824 to remove restrictions placed on Jinja2 given
https://github.com/spatialaudio/nbsphinx/issues/641. The issues
described there were resolved upstream of nbsphinx in
https://github.com/jupyter/nbconvert/pull/1737.
   - Lower bounds of 'nbconvert>=6.4.5' are not added as lower
     bound restrictions should be applied upstream of pyhf in
     nbsphinx. PR #1824 was applied only because it was
     necessary.

[codecov]

Codecov Report

Merging #1837 (d247e0b) into master (0693b9f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1837   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files          68       68           
  Lines        4315     4315           
  Branches      725      725           
=======================================
  Hits         4236     4236           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.44% <ø> (ø)
doctest	60.78% <ø> (ø)
unittests-3.10	96.03% <ø> (ø)
unittests-3.7	96.02% <ø> (ø)

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