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 one new rule being ignored (which weren't check before anyway):

• RET504: Unnecessary variable assignment before return statement, which we do everywhere as it increases readability.

[lucabello]

Looks like unit tests on this were broken; I've fixed some, but for the others I'm clueless so they're probably worth a separate ticket?