ForwardRef and GenConverter do not seem to work together

• cattrs version: 1.8.0 (attrs 21.2.0)
• Python version: 3.9.7
• Operating System: Windows 10

Description

When using recursive types (see example below), structure with GenConverter raises a StructureHandlerNotFoundError with the message "Unsupported type: ForwardRef('Y'). Register a structure hook for it"
.
The error does not happen with PEP 563 enabled. That means when we add 'from future import annotations' to the program and
replace y: typing.Optional['Y'] with y: typing.Optional[Y] everything works as expected.

What I Did

import typing
import cattr
import attr

@attr.define(auto_attribs=True)
class X:
    a: int
    b: float
    y: typing.Optional['Y']   #with PEP563 enabled:   y: typing.Optional[Y]   (that does not raise an exception) 

@attr.define(auto_attribs=True)
class Y:
    a: typing.List[int]
    x: X


converter = cattr.GenConverter()

data = { 'a': [1,2,3], 'x': {'a':1, 'b':2.000001, 'y': {'a': [], 'x': {'a': 5, 'b': 3.1, 'y': None}}} }
d = converter.structure( data, Y )  # <---- raises StructureHandlerNotFoundError    Unsupported type: ForwardRef('Y'). Register a structure hook for it

Start by running the supplied Python reproducer with cattrs 1.8.0 and trace the GenConverter structure path for the recursive ForwardRef('Y'). Compare this behavior with the PEP 563 variant; done means the original example structures successfully without requiring a manually registered hook.