Polish developer-facing CI automation

more automation to keep all linting / testing tools (black, flake8, tox, …) up to date and consistent
CI support for multiple python versions (and possibly some dependency versions)
faster CI for diffs (e.g., consider use of testmon to only run tests affected by changes)
investigation of more linters or flake8 plugins
cypress performance (parallel testing)

Migrated from T4084 (view on Phabricator)

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