issue26893: use mro() to examine class heirarchy

Lib/enum.py

@@ -124,7 +124,7 @@ class EnumMeta(type):
 
         # save attributes from super classes so we know if we can take
         # the shortcut of storing members in the class dict
-        base_attributes = {a for b in bases for a in b.__dict__}
+        base_attributes = {a for b in enum_class.mro() for a in b.__dict__}
 
         # Reverse value->name map for hashable values.
         enum_class._value2member_map_ = {}