B902: Add exceptions for standard library metaclasses

I faced a false positive when deriving my metaclass from ABCMeta, which should be fixed with this commit. I also added EnumMeta to this list, because it was the only other public metaclass in the standard library.

bugbear.py

@@ -1028,7 +1028,7 @@ def is_classmethod(decorators: Set[str]) -> bool:
             return
 
         bases = {b.id for b in cls.bases if isinstance(b, ast.Name)}
-        if "type" in bases:
+        if any(basetype in bases for basetype in ("type", "ABCMeta", "EnumMeta")):
             if is_classmethod(decorators):
                 expected_first_args = B902.metacls
                 kind = "metaclass class"