Previously was restricted to >=1.8.2,<2.1, but newer versions have some
nice improvements I'd like to be able to use in upcoming changes.
Changelog: https://www.sphinx-doc.org/en/master/changes.html#release-3-1-0-released-jun-08-2020
There are two issues that came up:
1. `highlightlang` is deprecated for `highlight`.
2. Doesn't like having two `automethod` generated for the same
`Metafunc.parametrize` method. Gives this warning:
`pytest/doc/en/reference.rst:846: WARNING: duplicate object description of _pytest.python.Metafunc.parametrize, other instance in reference, use :noindex: for one of them`
To work around this I make `pytest.mark.parametrize` link to
`Metafunc.parametrize` instead of repeating it.
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