python3: fix Abstract* __new__ usage

The __new__ method receives the same arguments as __init__.
object.__new__ accepts them if __new__ isn't overridden but rejects them
otherwise.

In Python3 though, if __new__ is overridden and then __bases__ is modified
so that __new__ isn't overridden anymore, object.__new__ will
erroneously reject arguments as if it were still overridden.
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