Fix sphinx error at tests execution

[sandcha]

Fixes the Extension error: Could not import extension sphinx.builders.latex (exception: cannot import name 'contextfunction' from 'jinja2' (/.../virtualenvs/doc-install/lib/python3.7/site-packages/jinja2/__init__.py)) raised at make test.

Additional information

• make test was executed in a virtual environment after make install.
• The error was detected with Jinja2 v.3.1.1, Sphinx v.1.5.1 and Flask 1.1.4.

[sandcha]

This is a draft pull request as:

• It fixes Jinja2 dependency revision but we might find a better solution as Jinja2 isn't a first level dependency but comes with Flask.
• The error is also related to Sphinx and, as said by this comment for a similar issue, it might be better to upgrade Sphinx dependency instead.
• With Jinja2 fix make install still contains this message:

  ERROR: pip's legacy dependency resolver does not consider dependency conflicts when selecting packages. This behaviour is the source of the following dependency conflicts.
  flake8 3.9.2 requires pycodestyle<2.8.0,>=2.7.0, but you'll have pycodestyle 2.8.0 which is incompatible.
  flask 1.1.4 requires Jinja2<3.0,>=2.10.1, but you'll have jinja2 3.0.3 which is incompatible.

Jinja2 releases history.
Sphinx releases history.

[MattiSG]

I'm in favour of upgrading Sphinx if that fixes the issue 🙂

[MattiSG]

Superseded by #264.