B024 should be satisfied by class attribute declarations like `attr: ClassVar[T]`

[Zac-HD]

In addition to traditional abstract methods, ABC could just be there to declare a class variable that should be set for concrete classes in a less verbose way than using annotations and properties:

class Abstract(ABC):
    attr: ClassVar[str]

This is checked by mypy and will also raise an AttributeError if trying to access attr on a concrete class that doesn't set it. See https://peps.python.org/pep-0526/

Originally posted by @alk-gpajot in #278 (comment)