Fix broken CI checks

[aphedges]

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

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CI was failing for 03404c8 because of two problems:

• Black was failing from Incompatible with click 8.1.0 (ImportError: cannot import name '_unicodefun' from 'click') psf/black#2964. Updating Black to the latest version fixes it without reformatting any of the code.
• Sphinx was failing from a change in Jinja 3.1.0 that interacted with the old version of Sphinx in this repository. This affected CPython itself earlier this week: https://bugs.python.org/issue47138. I'm pinning Jinja2 for now because the Sphinx upgrade will be more difficult.