Fixed #13206 -- call super().__init__() in Model.__init__ to allow mixins to do things there.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15317 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/modeltests/model_inheritance/models.py

@@ -143,3 +143,11 @@ class Copy(NamedURL):
 
     def __unicode__(self):
         return self.content
+
+class Mixin(object):
+    def __init__(self):
+        self.other_attr = 1
+        super(Mixin, self).__init__()
+
+class MixinModel(models.Model, Mixin):
+    pass