Fix 'stack smashing' bug

Was using the 'n' format character which expects a Py_ssize_t*, but was passing an int*.
Switch to using the 'i' character.

test/test_extension/slots_test.c

@@ -15,7 +15,7 @@ slots_tester_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
 
     int n;
-    if (!PyArg_ParseTuple(args, "n", &n))
+    if (!PyArg_ParseTuple(args, "i", &n))
         return NULL;
 
     /* create attrgetterobject structure */