Revert "Treat ModelAdmin like an old python class"

This reverts commit 6695582.

Author: macro1

simple_history/admin.py

@@ -24,7 +24,7 @@
 USER_NATURAL_KEY = tuple(key.lower() for key in USER_NATURAL_KEY.split('.', 1))
 
 
-class SimpleHistoryAdmin(admin.ModelAdmin, object):
+class SimpleHistoryAdmin(admin.ModelAdmin):
     object_history_template = "simple_history/object_history.html"
     object_history_form_template = "simple_history/object_history_form.html"