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.

Python/traceback.c

@@ -87,7 +87,7 @@ tb_new_impl(PyTypeObject *type, PyObject *tb_next, PyFrameObject *tb_frame,
 }
 
 static PyObject *
-tb_dir(PyTracebackObject *self)
+tb_dir(PyTracebackObject *self, PyObject *Py_UNUSED(ignored))
 {
     return Py_BuildValue("[ssss]", "tb_frame", "tb_next",
                                    "tb_lasti", "tb_lineno");