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].

Objects/funcobject.c

@@ -581,7 +581,7 @@ function_call(PyObject *func, PyObject *args, PyObject *kwargs)
     PyObject **stack;
     Py_ssize_t nargs;
 
-    stack = &PyTuple_GET_ITEM(args, 0);
+    stack = _PyTuple_ITEMS(args);
     nargs = PyTuple_GET_SIZE(args);
     return _PyFunction_FastCallDict(func, stack, nargs, kwargs);
 }