Validate Field(validators=[...]) more precisely

[UnknownPlatypus]

I have made things!

Make _ValidatorCallable more strict by adding a typevar and binding it to the getter type of the various fields:

-_ValidatorCallable: TypeAlias = Callable[[Any], None] # noqa: PYI047
+_VT = TypeVar("_VT", contravariant=True)
+_ValidatorCallable: TypeAlias = Callable[[_VT], None] 

Fixes #264

[github-actions]

Diff from mypy_primer, showing the effect of this PR on type check results on a corpus of open source code:

zulip (https://github.com/zulip/zulip)
+ zerver/models/realm_emoji.py:34: error: Argument "validators" to "TextField" has incompatible type "list[_Deconstructible]"; expected "Iterable[Callable[[Never], None]]"  [arg-type]

[UnknownPlatypus]

• zerver/models/realm_emoji.py:34: error: Argument "validators" to "TextField" has incompatible type "list[_Deconstructible]"; expected "Iterable[Callable[[Never], None]]" [arg-type]

This is very interesting, because we now have _GT in the init signature, mypy tries to resolve the typevar from it and seem to fail and pick Never.

I think it would work if we managed to remove the custom _pyi_private_get_type and replace _GT with typevar + defaults like TypeVar("_StrFieldGT", covariant=True, default=str) -> cf #1264