Simplify calling.

Objects/classobject.c

@@ -1072,21 +1072,15 @@ static PyMappingMethods instance_as_mapping = {
 static PyObject *
 instance_item(PyInstanceObject *inst, Py_ssize_t i)
 {
-	PyObject *func, *arg, *res;
+	PyObject *func, *res;
 
 	if (getitemstr == NULL)
 		getitemstr = PyString_InternFromString("__getitem__");
 	func = instance_getattr(inst, getitemstr);
 	if (func == NULL)
 		return NULL;
-	arg = Py_BuildValue("(n)", i);
-	if (arg == NULL) {
-		Py_DECREF(func);
-		return NULL;
-	}
-	res = PyEval_CallObject(func, arg);
+	res = PyObject_CallFunction(func, "n", i);
 	Py_DECREF(func);
-	Py_DECREF(arg);
 	return res;
 }