[1.7.x] Use the new implementation of `six.with_metaclass`.

No more `NewBase` horrors.

Thanks to bendavis78 for his work on merging this into six.

Backport of a2340ac6d6 from master

django/db/models/base.py

@@ -62,19 +62,9 @@ class ModelBase(type):
     def __new__(cls, name, bases, attrs):
         super_new = super(ModelBase, cls).__new__
 
-        # six.with_metaclass() inserts an extra class called 'NewBase' in the
-        # inheritance tree: Model -> NewBase -> object. But the initialization
-        # should be executed only once for a given model class.
-
-        # attrs will never be empty for classes declared in the standard way
-        # (ie. with the `class` keyword). This is quite robust.
-        if name == 'NewBase' and attrs == {}:
-            return super_new(cls, name, bases, attrs)
-
         # Also ensure initialization is only performed for subclasses of Model
         # (excluding Model class itself).
-        parents = [b for b in bases if isinstance(b, ModelBase) and
-                not (b.__name__ == 'NewBase' and b.__mro__ == (b, object))]
+        parents = [b for b in bases if isinstance(b, ModelBase)]
         if not parents:
             return super_new(cls, name, bases, attrs)
 

django/utils/six.py

@@ -628,7 +628,21 @@ _add_doc(reraise, """Reraise an exception.""")
 
 def with_metaclass(meta, *bases):
     """Create a base class with a metaclass."""
-    return meta("NewBase", bases, {})
+    # This requires a bit of explanation: the basic idea is to make a
+    # dummy metaclass for one level of class instantiation that replaces
+    # itself with the actual metaclass.  Because of internal type checks
+    # we also need to make sure that we downgrade the custom metaclass
+    # for one level to something closer to type (that's why __call__ and
+    # __init__ comes back from type etc.).
+    class metaclass(meta):
+        __call__ = type.__call__
+        __init__ = type.__init__
+        def __new__(cls, name, this_bases, d):
+            if this_bases is None:
+                return type.__new__(cls, name, (), d)
+            return meta(name, bases, d)
+    return metaclass('temporary_class', None, {})
+
 
 def add_metaclass(metaclass):
     """Class decorator for creating a class with a metaclass."""