bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS. (GH-6030)

METH_NOARGS functions need only a single argument but they are cast
into a PyCFunction, which takes two arguments.  This triggers an
invalid function cast warning in gcc8 due to the argument mismatch.
Fix this by adding a dummy unused argument.

Doc/extending/newtypes_tutorial.rst

@@ -457,7 +457,7 @@ We define a single method, :meth:`Custom.name()`, that outputs the objects name
 concatenation of the first and last names. ::
 
    static PyObject *
-   Custom_name(CustomObject *self)
+   Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
    {
        if (self->first == NULL) {
            PyErr_SetString(PyExc_AttributeError, "first");