Support `zuban`

[A5rocks]

I haven't tried zuban out yet, but hopefully being mypy compatible means minimal work for us!

[A5rocks]

Blocked on zubanls/zuban#483 and zubanls/zuban#484.

[jakkdl]

(both blockers have now been resolved)

[A5rocks]

Yeah, unfortunately there's been no release since then :(

(I could probably compile from source but ehhh, not worth it :^)

[A5rocks]

Now just zubanls/zuban#518 and then I think everything under src/trio/_core/_tests/type_tests works. (I haven't checked src/trio/_tests/type_tests)

We need to make a couple changes because zuban recognizes mypy error codes but the inference logic is slightly different, but they're pretty simple to do.