False positive B024 - custom metaclass derived from ABCMeta treated as abstract base class

[antonymayi]

Another false positive with the recent B024 is when implementing custom metaclass extended from ABCMeta - they shouldn't be classified as abstract base classes at all:

class MyMeta(abc.ABCMeta):  # THIS GETS FALSE POSITIVE B024 BUT IS PERFECTLY LEGIT
    def __new__(mcs, name, bases, namespace):
        #  whatever custom metaclass magic here
        return super().__new__(mcs, name, bases, namespace)

class MyAbstract(metaclass=MyMeta):
    @abstractmethod
    def foobar(self):
        pass

B024 MyMeta is an abstract base class, but it has no abstract methods. Remember to use @abstractmethod, @abstractclassmethod and/or @abstractproperty decorators.
class MyMeta(abc.ABCMeta):

-> MyMeta is actually not an abstract base class (instead, it is a custom metaclass for creating abstract base classes) so this is false positive.

[jakkdl]

Hmm, should one maybe disable b024 for ABCMeta and only do it with ABC?
@Zac-HD

[Zac-HD]

Ah, we should only warn on ABCMeta when it's a keyword argument (with metaclass=), and for ABC when that's a positional argument. is_abc_class() is currently a bit too broad.