Fix failure to identify an unresolvable classvar in PEP-649 annotations.

[DavidCEllis]

Fixes an issue where an annotation that could not resolve as a ForwardRef would not be correctly identified as a ClassVar.

This could be an issue if typing aliases were ever removed for instance. This example would previously fail to detect CV as a ClassVar as the AttributeError causes the annotation to become an opaque forward reference.

Using reannotate this now correctly extracts and evaluates the ClassVar.

import typing as t
from ducktools.classbuilder.prefab import prefab

@prefab
class Example:
    CV: t.ClassVar[t.RemovedAlias] = ['a']