ci: support mypy 0.9xx

[henryiii]

I'd recommend running mypy from pre-commit (or maybe something like nox) as it's very important to fully control MyPy's environment, and usually nice to have it pinned as well (say with pre-commit). (See the recommendation here, for an example: https://scikit-hep.org/developer/style#type-checking )

[FFY00]

I don't want it on pre-commit as mypy can take a while to run, but putting it on nox makes sense.

[henryiii]

You can set it to only run in the manual stage, which only runs when requested. There's an example of that under check-manifest. That way you still get the auto-updates from pre-commit.ci, for example. Feel free to do whatever you want there, though. :)

MyPy is on the very border of what I'd call a pre-commit stage check, but since failing it is invalid, I usually leave it on the normal pre-commit stage. Check-manifest is clearly on the other side of that border - slow and you very rarely ever break it. I break mypy all the time. :)

Anyway, looks like there's a small type issue. Do you have it or would you like me to fix it?

[henryiii]

Looks easy, I'll put an example fix in.

[FFY00]

This ended up being fixed separately, but thank you so much!