"None" オブジェクト
*******************

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 and is immortal.

バージョン 3.12 で変更: "Py_None" is immortal.

Py_RETURN_NONE

   Return "Py_None" from a function.
