Configure and apply ruff formatter rules

[daniil-berg]

Since we already use the ruff linter extensively, we might as well utilize its formatter. The configuration should be fairly minimal and applying its black-like formatting could be done in a single commit.

We should then integrate the format checker into the CI workflow and thereby ensure consistent code style from that point forward. The isort-like import sorting should be integrated as well. All of it should apply to both src/ and tests/.