Scratch that, my revised idea is fundamentally broken, as this example illustrates:

>>> class BaseClass: pass
... 
>>> class OtherClass: pass
... 
>>> class SubClass(OtherClass, BaseClass): pass
... 
>>> SubClass.mro()
[<class '__main__.SubClass'>, <class '__main__.OtherClass'>, <class '__main__.BaseClass'>, <class 'object'>]


The PEP as written handles this correctly, while the alternative signature would fail miserably ("OtherClass" wouldn't chain up to "BaseClass" properly). So I'll review the rest of the patch, and we can figure out the documentation problem later.