`typing.get_type_hints` cannot resolve forward references in `cls.__init__` annotations

The is related to #593/#760. Because #760 populates the generated __init__ method's globals at the time that the class is created, new globals aren't visible to get_type_hints by default.

Example code demonstrating this:

import attrs
import typing

@attrs.define
class HasForwardRef:
    ref: "ClassDefinedLater"

class ClassDefinedLater:
    pass

typing.get_type_hints(HasForwardRef.__init__)

This last call raises an error.

I've got a workaround for this in the code where I hit this, but it really wasn't obvious at first what was going on.

Start by running the issue's example and trace how attrs generates HasForwardRef.init and populates its globals when the class is created. Investigate how typing.get_type_hints resolves that method's forward references; done means the shown call succeeds after ClassDefinedLater is defined, with regression coverage for the example.