Special-case setting __new__ in class

[jdemeyer]

Python special-cases the name __new__ in a class: if it's a Python function, it's automatically wrapped in a staticmethod:

>>> class X:
...     def __new__(cls): return object.__new__(cls)
>>> X.__dict__["__new__"]
<staticmethod object at 0x7f0d52eafb38>

This PR implements this feature for Cython functions.