revert: Remove Jinja2 restrictions given nbconvert v6.4.5 (#1837)

* Revert PR #1824 to remove restrictions place on Jinja2 given
spatialaudio/nbsphinx#641. The issues
described there were resolved upstream of nbsphinx in
jupyter/nbconvert#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.

setup.py

@@ -55,7 +55,6 @@
             'sphinx-click',
             'sphinx_rtd_theme',
             'nbsphinx!=0.8.8',  # c.f. https://github.com/spatialaudio/nbsphinx/issues/620
-            'Jinja2!=3.1.0',  # c.f. https://github.com/spatialaudio/nbsphinx/issues/641
             'ipywidgets',
             'sphinx-issues',
             'sphinx-copybutton>=0.3.2',