bpo-40522: _PyThreadState_Swap() sets autoTSSkey (GH-19939)

In the experimental isolated subinterpreters build mode,
_PyThreadState_GET() gets the autoTSSkey variable and
_PyThreadState_Swap() sets the autoTSSkey variable.

* Add _PyThreadState_GetTSS()
* _PyRuntimeState_GetThreadState() and _PyThreadState_GET()
  return _PyThreadState_GetTSS()
* PyEval_SaveThread() sets the autoTSSkey variable to current Python
  thread state rather than NULL.
* eval_frame_handle_pending() doesn't check that
  _PyThreadState_Swap() result is NULL.
* _PyThreadState_Swap() gets the current Python thread state with
  _PyThreadState_GetTSS() rather than
  _PyRuntimeGILState_GetThreadState().
* PyGILState_Ensure() no longer checks _PyEval_ThreadsInitialized()
  since it cannot access the current interpreter.