Enable Ruff flake8-async (ASYNC)

[Avasam]

Ref #13295
flake8-async (ASYNC)

A handful of async-related rules that look reasonable. We happen to already respect all of them except 1: run-process-in-async-function (ASYNC221)

I followed the rule's recommendation using anyio for the sole reason that it's lighter than trio, and it's better than writing something like the followign (unless we add it to our internal lib):

But I don't know what's the actual impacts, if any. Can someone else tell me if async subprocess call in async method has real benefits or if we should just ignore that rule?

[srittau]

Apart from the fact that asyncio in Python is ... special (why aren't there any stdlib variants of those?): We don't need async calls in main(), considering that nothing is running to parallel at this point.

[AlexWaygood]

yeah, not sure that this group adds much value for this repo. The changes here are the kind of thing that would be important to ensure things stay performant for a large asynchronous program, but all our async scripts are pretty small and not really performance-critical. As @srittau points out, the extra await calls in main() don't offer any benefit in this case, since everything is running synchronously in this script until the for task in asyncio.as_completed(tasks): loop lower down in the main() function.