PEP621 / pyproject.toml for easier optional dependencies

[tony]

via: https://stackoverflow.com/a/72666457/1396928

There may be other ways, I've used requirements/dev.txt back in the day. Also there's imply adding an extras_require to setup.py

https://peps.python.org/pep-0621/

This would make it easier to declare optional dependencies, e.g. flake8, black

I personally don't quite use this (since I'm on poetry, it's pyproject.toml and slightly different)