adopt pyproject.toml

[ThomasWaldmann]

Can be done in 2 steps:

• there should be a minimal pyproject.toml in any case, giving build system requirements and backend
• even better, move all metadata from setup.cfg / .py to pyproject.toml (for moin as well as for the misc. other tools used for development)

https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/

Example PR from borg: borgbackup/borg#7997

[ThomasWaldmann]

flake8 is a bit of a troublemaker here as its settings can't live in pyproject.toml, but there is ruff as a nice replacement.

as this tends to be a slightly bigger change, it is best done as a separate PR, as seen there:

borgbackup/borg#7808

[UlrichB22]

Recommendation from Thomas in #1570:

Replace flake8 by ruff - it integrates better into pyproject.toml.