gh-123923: Defer refcounting for f_executable in _PyInterpreterFrame (#123924)

Use a `_PyStackRef` and defer the reference to `f_executable` when
possible. This avoids some reference count contention in the common case
of executing the same code object from multiple threads concurrently in
the free-threaded build.

Python/legacy_tracing.c

@@ -143,9 +143,8 @@ _PyEval_SetOpcodeTrace(
     bool enable
 ) {
     assert(frame != NULL);
-    assert(PyCode_Check(frame->f_frame->f_executable));
 
-    PyCodeObject *code = (PyCodeObject *)frame->f_frame->f_executable;
+    PyCodeObject *code = _PyFrame_GetCode(frame->f_frame);
     _PyMonitoringEventSet events = 0;
 
     if (_PyMonitoring_GetLocalEvents(code, PY_MONITORING_SYS_TRACE_ID, &events) < 0) {