super doesn't work on PersistentMapping

[Estartu]

I have a Class inheriting from three base classes. First
PersistentMapping and two of my own classes.

In init of this class I call super(MyClass, self).init() but the
init methods of my two classes get never called.

If i move PersistentMapping to inherit last. The init methods of
both classes are called.

[tseaver]

See https://gist.github.com/tseaver/8336917

[tseaver]

This behavior is due to implementing PersistentMapping by deriving from the stdlib's (Iterable)UserDict, whose __init__() does not call super() in either Python 2 or Python 3. We can't fix that problem.