Includes some related changes: support a local venv by excluding it from
Git and Flake8, and fix things that regressed over the past half year
since we weren’t pinning dependencies yet. (CI failed on the previous
commit, and the commit before that was in February.)

Note that I ran pip-compile on Toolforge’s Python 3.9 – experience with
other tools has taught me that pip-compile on my desktop’s Python 3.10
will omit a package that’s required on Toolforge.