let proxy models multiple-inherit from the same concrete base model

[Ryan Kelly]

Currently proxy models are required to have a single concrete base model class. Unfortunately this prevents me from combining several proxy subclasses of the same model. In my particular use case, I have two different apps that provide two different proxies of the standard User model. To successfully use both apps I need to create another subclass that combines the two, e.g:

class MyUser(App1User,App2User):
    class Meta:
        proxy = True

This gives a TypeError: "Proxy model 'MyUser' has more than one non-abstract model base class". But since App1User and App2User proxy the same underlying model, there's no ambiguity introduced by this multiple inheritance and I think it should be permitted.

Attached is a simple patch to make this work, by permitting additional concrete base classes if they are identical to the one that was already found.