Replace call to add_stylesheet with add_css_file

[befeleme]

This should fix documentation build failures with Sphinx 4.

[codecov]

Codecov Report

Merging #277 (835bb2b) into master (867c4af) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #277   +/-   ##
=======================================
  Coverage   98.90%   98.90%           
=======================================
  Files           3        3           
  Lines         273      273           
  Branches       46       46           
=======================================
  Hits          270      270           
  Partials        3        3           

[njsmith]

Ugh thank you, tacking down sphinx regressions is the worst.

[trio-bot]

Hey @befeleme, it looks like that was the first time we merged one of your PRs! Thanks so much! 🎉 🎂

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[pquentin]

@befeleme Thank you! Would you happen to be interested in opening a pull request that tests Sphinx 4? The CI is still only testing Sphinx 3 and earlier

https://github.com/python-trio/sphinxcontrib-trio/blob/master/.github/workflows/ci.yml