original' attribute if it is invoked with
        an object.  If it is invoked with None, it should return a false
        class-method emulator instead.

        For some reason, Python's documentation recommends to define
        descriptors' __get__ methods with the 'type' parameter as optional,
        despite the fact that Python itself never actually calls the descriptor
        that way.  This is probably do to support 'foo.__get__(bar)' as an
        idiom.  Let's make sure that the behavior is correct.  Since we don't
        actually use the 'type' argument at all, this test calls it the
        idiomatic way to ensure that signature works; test_proxyInheritance
        verifies the how-Python-actually-calls-it signature.
        c