Improve developer-facing 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

comments:

• TODO: make a list of well defined subtasks which can be estimated.

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Migrated from T3958 (view on Phabricator)