Revert "Treat manager like an old-style class"

This reverts commit c5ed822.

Author: macro1

simple_history/manager.py

@@ -13,7 +13,7 @@ def __get__(self, instance, owner):
         return HistoryManager(self.model, instance)
 
 
-class HistoryManager(models.Manager, object):
+class HistoryManager(models.Manager):
     def __init__(self, model, instance=None):
         super(HistoryManager, self).__init__()
         self.model = model