Support for classes that define __new__ for mock_constructor

[fornellas]

It currently has no support for classes with __new__, and will reject patching those.

IIRC, it was tough getting around Python idiosyncrasies to make mock_constructor work, and I willingly decided not to implement __new__ at first to simplify things, but this is potentially doable.

[fornellas]

@deathowl I just remembered there's an in-depth documentation of the trickery behind mock_constructor here. I think things should work, as we're not touching __new__ from the class, would have to test things.