Type annotation for resolve_types isn't correct

```
import attr

def test(cls: type) -> None:
    if attr.has(cls):
        attr.resolve_types(cls)
```

Fails with `foo.py:6: error: Value of type variable "_A" of "resolve_types" cannot be "Type[AttrsInstance]"  [type-var]`

I think the easy fix is 

```
_A = TypeVar("_A", bound=type[AttrsInstance])
```

