the case where an abstractmethod decorator is used on a class that does not subclass abc.ABC uses the ABC metaclass, is probably a bug. It would be great to have a rule to detect the obvious case where it is used on the wrong type of class (when all ancestors of the class in the same file for instance).

If there is a rule that already implements this, let me know. :)

Here is a real example from PyTorch:

class RemoteCacheBackend:
    """
    A backend implementation for accessing a remote/distributed cache.
    """

    def __init__(self, cache_id: str) -> None:
        pass

    @abstractmethod
    def get(self, key: str) -> Optional[object]:
        pass

    @abstractmethod
    def put(self, key: str, data: bytes) -> None:
        pass