use ruff instead of flake8

[lucabello]

The flake8 plugins we use are re-implemented in ruff, which is why they have been removed from the dependencies.

isort is enabled (and used when formatting) through the I001 rule.

The "exclude list" has been updated to omit folders that ruff excludes by default.

Everything else is the same, the only difference being two new rules being ignored (which weren't checked before anyway):

• C901: Function is too complex, which I excluded as we were not checking it before either (an eventual refactor should have its own PR);
• RET504: Unnecessary variable assignment before return statement, which we do everywhere as it increases readability.

[lucabello]

@simskij you might need to review the branch protection rules on this repo, as it looks like it's stuck even though all checks passed :)