Use ruff for linting

Use ruff instead of flake8. Aside from being substantially faster, ruff
includes a lot of flake's plug-ins built-in, some of which may be useful
to enable as a follow-up.

Also move the relevant configuration out of the script in .ci/ and into
the project-wide settings. A side effect of this is that IDEs should
detect these settings, and ignore the same rules as CI, avoiding false
positives while developing locally.

Reviewed-by: Oliver Smith <ollieparanoid@postmarketos.org>
Reviewed-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230529203448.18656-1-hugo@whynothugo.nl%3E