Fix classmethod for use in cdef classes.

Fix classmethod for use in cdef classes.

The issue here (as pointed out by Thomas Hunger) is that once the CClass
body gets executed the class is already constructed, and its methods
are actual methods not functions (as in Python).

The CClassScope now returns a utility function on lookup of "classmethod"
that creates a class method out of a method (rather than out of a function).
When added to the type dictionary, the result is exactly the same as setting
the METH_CLASS flag.