"None" 객체
***********

Note that the "PyTypeObject" for "None" is not directly exposed in the
Python/C API.  Since "None" is a singleton, testing for object
identity (using "==" in C) is sufficient. There is no "PyNone_Check()"
function for the same reason.

PyObject *Py_None

   The Python "None" object, denoting lack of value.  This object has
   no methods. It needs to be treated just like any other object with
   respect to reference counts.

Py_RETURN_NONE

   Properly handle returning "Py_None" from within a C function (that
   is, increment the reference count of "None" and return it.)
