instance_of validator cannot refer to the current class

Hi,

I'm working on a project where it's useful for an attribute to refer to the current class. Currently, I'm using something like:

from __future__ import annotations  # Needed for recursive type annotations.


@attr.s(slots=True)
class Namespace(object):
    parent: Namespace = attr.ib(
        validator=lambda inst, attr_, val: attr.validators.instance_of(Namespace)(inst, attr_, val)
    )

Is there currently a better way to do this?
If not, would you be open to a pull request that might allow something like this?

validator = attr.validators.instance_of(type_getter=lambda: Namespace)

(Also, if you have other suggestions for an approach, I'd be happy to implement those in a PR instead.)

Start by reading the validator implementation and its existing tests, then trace how instance_of resolves the expected type. Compare the proposed type_getter approach with the current lambda workaround; done means current-class references can be validated without the workaround and existing behavior remains covered.