Allow #[new] to return arbitrary objects

[davidhewitt]

Since #3287 #[new] is now able to return Py<T> to return an existing Python object (e.g. for singleton classes).

In pure-python code I believe the return value of __new__ is not restricted at all, so it may be elegant in PyO3 if similarly any T: IntoPy<PyObject> could be returned from #[new]. This should be possible as a pure addition without backwards-compatibility concerns.