bpo-35199: Add an internal _PyTuple_ITEMS() macro (GH-10434)

* _PyTuple_ITEMS() gives access to the tuple->ob_item field and cast the
  first argument to PyTupleObject*. This internal macro is only usable if
  Py_BUILD_CORE is defined.
* Replace &PyTuple_GET_ITEM(ob, 0) with _PyTuple_ITEMS(ob).
* Replace PyTuple_GET_ITEM(op, 1) with &_PyTuple_ITEMS(ob)[1].

Python/ceval.c

@@ -4662,7 +4662,7 @@ do_call_core(PyObject *func, PyObject *callargs, PyObject *kwdict)
             }
 
             C_TRACE(result, _PyCFunction_FastCallDict(func,
-                                                      &PyTuple_GET_ITEM(callargs, 1),
+                                                      &_PyTuple_ITEMS(callargs)[1],
                                                       nargs - 1,
                                                       kwdict));
             Py_DECREF(func);