Warn about unused `noqa`s

[A5rocks]

Following the advice in the docs, I marked ASYNC as external to ruff. This also means I don't get warned about unnecessary noqas, unless I'm missing something. It's possible the docs describe how to avoid this! I didn't really read them.

[jakkdl]

No iirc you're correct and we don't warn about unused noqas. If you run it through flake8 you can use https://github.com/plinss/flake8-noqa (think we've had problems with compatibility in the past, but AFAIK it should work now), but we probably want a solution for standalone usage.