Various minor things

[jakkdl]

1. I thought it was somewhat doomed to use ruff+flake8-async, but after way too long stumbled upon https://docs.astral.sh/ruff/settings/#lint_external We should probably suggest using this in docs.

2. flake8 --per-file-ignores works with our 5-character error codes... for now. We should add a test to see when they get around to running validation on it and break compatibility in another way.

   • I also started looking at adding an --per-file-disable to get around it and for running as standalone, and it should be fairly doable

3. should add an example on how to run flake8-async through flake8 in pre-commit.

4. fix visitor102: https://github.com/python-trio/flake8-async/pull/307/files#diff-4852409e79e4eb0f3736b0d3c6b6bfd8c2a14411b92416bc203a387dd6986829