Add ruff linter and formatter

[CSchoel]

Nolds already has a .ruff.toml. After #43, we can move the content to pyproject.toml. We should also double-check which settings we want and which rules we want to enable.

• Move content of .ruff.toml to pyproject.toml.
• Double check settings.
• Check what is the maximum number of rules that we can allow without creating too much hassle.
• Run ruff formatter over the whole codebase.
• Run all possible autofixes over the whole codebase.
• Check that unit tests still work.