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/codeobject.c

@@ -39,7 +39,7 @@ intern_strings(PyObject *tuple)
         if (v == NULL || !PyUnicode_CheckExact(v)) {
             Py_FatalError("non-string found in code slot");
         }
-        PyUnicode_InternInPlace(&PyTuple_GET_ITEM(tuple, i));
+        PyUnicode_InternInPlace(&_PyTuple_ITEMS(tuple)[i]);
     }
 }