typing: allow union types for instance_of

[hynek]

fixes #1336

[hynek]

Tiiiiin, is this fixableeeee?

tests/typing_example.py:231: error: Incompatible types in assignment (expression has type "UnionType", variable has type "int | C | str")
[assignment]
        kk: int | C | str = attr.ib(
                            ^
Found 1 error in 1 file (checked 1 source file)

Or are we doing typing bankruptcy (aka Any)?

[Tinche]

I played around with this a little. I don't think the type system can do this. The overload should be:

@overload
def instance_of(type: UnionType) -> _ValidatorType[Any]: ...