__Pyx_Raise(): mark cause with CYTHON_UNUSED

Cython/Compiler/Nodes.py

@@ -8116,7 +8116,8 @@ static void __Pyx_Raise(PyObject *type, PyObject *value, PyObject *tb, PyObject
 """,
 impl = """
 #if PY_MAJOR_VERSION < 3
-static void __Pyx_Raise(PyObject *type, PyObject *value, PyObject *tb, PyObject *cause) {
+static void __Pyx_Raise(PyObject *type, PyObject *value, PyObject *tb,
+                        CYTHON_UNUSED PyObject *cause) {
     /* cause is unused */
     Py_XINCREF(type);
     Py_XINCREF(value);