Unpin `sphinx` upper bound

[mxr]

CI fails with ImportError: cannot import name 'environmentfilter' from 'jinja2' (example).

Bumping Sphinx as described in https://github.com/sphinx-doc/sphinx/issues/10291

[mxr]

The tox docs step fails with

ERROR: InvocationError for command /home/runner/work/pytest/pytest/.tox/docs/bin/sphinx-build -W --keep-going -b html doc/en doc/en/_build/html -t changelog_towncrier_draft (exited with code 1)

I think this is from the below warning (since -W makes warnings into errors)

/home/runner/work/pytest/pytest/src/_pytest/pytester.py:docstring of _pytest.pytester.Pytester.TimeoutExpired:1: WARNING: duplicate object description of _pytest.pytester.Pytester.TimeoutExpired, other instance in reference/reference, use :noindex: for one of them

It seems there was some work to fix that in https://github.com/pytest-dev/pytest/issues/8730#issuecomment-860059789 but no change was merged

Beyond that, CI will still be red because of https://github.com/psf/black/issues/2964

[The-Compiler]

This seems like a duplicate of #9248.

[bluetech]

Indeed, we're handling this in #9248.