Future linting improvements

[deltragon]

Followup to #615.

• Forbid + for string concatenation
• Sort/format imports
  Needs to figure out how to deal with gi.require_version calls mixed in
• Improve doc comments, remove "obvious" ones
  this one might need to go into the style guide, if we can't check it automatically
• Use constants instead of magic values - https://docs.astral.sh/ruff/rules/magic-value-comparison/