Detect tasks that could disappear mid-execution

[alicederyn]

The docs for asyncio.create_task say:

Save a reference to the result of this function, to avoid a task disappearing mid-execution. The event loop only keeps weak references to tasks. A task that isn’t referenced elsewhere may get garbage collected at any time, even before it’s done.

It's really easy to miss this and try to fire-and-forget. It would be great if flake8-async detected this.

[cooperlees]

+1 on this. Would be smart we recommend people keep variables around until tasks complete etc. and link to these docs.

As always, please update README + unittests.

[Zac-HD]

Just a note that asyncio-specific checks should only emit warnings if asyncio is imported into the code we're linting.

(also applies to trio or curio fwiw, though I pretty quickly started a separate flake8-trio project for the former)

[jakkdl]

@Zac-HD this one might make sense to add to flake8-trio, is trivial to implement, and is supported by ruff (and would make more sense there to be moved under ASYNCxxx than as RUF006).

Checked the other two open issues, and they're already covered in flake8-trio.