gen_throw(): The caller doesn't own PyArg_ParseTuple()

"O" arguments, so must not decref them.  This accounts
for why running test_contextlib.test_main() in a loop
eventually tried to deallocate Py_None.

Objects/genobject.c

@@ -217,10 +217,8 @@ gen_throw(PyGenObject *gen, PyObject *args)
 
 	/* First, check the traceback argument, replacing None with
 	   NULL. */
-	if (tb == Py_None) {
-		Py_DECREF(tb);
+	if (tb == Py_None)
 		tb = NULL;
-	}
 	else if (tb != NULL && !PyTraceBack_Check(tb)) {
 		PyErr_SetString(PyExc_TypeError,
 			"throw() third argument must be a traceback object");