fix "...has no attribute 'sub_and_superclass_dict'". Thanks to Mattias Brändström.

this occurred if a subclass defined __init__ and accessed class
members before calling the superclass __init__
(__getattribute__ had a problem, as "sub_and_superclass_dict" was set in __init__).

pexp/models.py

@@ -34,3 +34,4 @@ if not (django_VERSION[0]<=1 and django_VERSION[1]<=1):
         field2 = models.CharField(max_length=10)
     class Model2C(Model2B):
         field3 = models.CharField(max_length=10)
+