For your consideration: a patch which implements the suggestion I made here:
pydantic#4101

Briefly: pydantic automatically wraps validator methods using
`@classmethod`. Hence the first argument to a user-provided validator
should be `cls`. But mypy doesn't know this: it analyses validator
methods like any other regular method, believing the first parameter
`cls` to be a model instance (usually denoted self).

This means that if one annotates `cls` as `Type[...]` then mypy
believes raises an error:

	error: The erased type of self "Type[temp.Model]" is not a supertype of its class "temp.Model"

I concede that this is an extremely niche thing. The only tangible
end-user benefit I can think of is that it'll stop you from calling
instance methods in a validator.

----

I haven't written a mypy plugin before, so this is a bit of a
hack-until-it-works. But it was more straightforward than I expected to
get something working!