bpo-36356: Destroy the GIL at exit (GH-12453)

* Add _PyEval_FiniThreads2(). _PyEval_FiniThreads() now only clears
  the pending lock, whereas _PyEval_FiniThreads2() destroys the GIL.
* pymain_free() now calls _PyEval_FiniThreads2().
* Py_FinalizeEx() now calls _PyEval_FiniThreads().