style: Enable checking for pep8-naming (N) rules with ruff, fix N804, N805 and N813

[echoix]

This PR initially wanted to enable and fix N804, but some similar issues could be solved at once.

So, this PR enables pep8-naming (N) rules with ruff.

Rules already respected:

• dunder-function-name (N807)
• constant-imported-as-non-constant (N811)
• camelcase-imported-as-acronym (N817)

Fixed rules:

• invalid-first-argument-name-for-class-method (N804) -> 2 instances
  • Ruff rule: https://docs.astral.sh/ruff/rules/invalid-first-argument-name-for-class-method/
  • Two files had class methods that didn't use cls as their first argument when they should have been
  • Can be autofixed for further errors in the future
• invalid-first-argument-name-for-method (N805) -> 16 instances
  • Counterpart of the previous N804, but for classes not using self as the first argument when they should
  • Ruff rule: https://docs.astral.sh/ruff/rules/invalid-first-argument-name-for-method/
  • Three instances in gui/wxpython/mapswipe/frame.py were ignored, as an inner class is defined using self2 and also references self from the containing class. All other instances were misusages in test files.
  • Can be autofixed for further errors in the future
• camelcase-imported-as-lowercase (N813) -> 1 instance
  • Ruff rule: https://docs.astral.sh/ruff/rules/camelcase-imported-as-lowercase/
  • One instance only in a pytest file -> used the non-aliased name directly, not much different