bpo-20526: Fix PyThreadState_Clear(): don't decref frame (GH-19120) (GH-19136) (GH...

bpo-20526: Fix PyThreadState_Clear(): don't decref frame (GH-19120) (GH-19136) (GH-19137)

PyThreadState.frame is a borrowed reference, not a strong reference:
PyThreadState_Clear() must not call Py_CLEAR(tstate->frame).

Remove test_threading.test_warnings_at_exit(): we cannot warranty
that the Python thread state of daemon threads is cleared in a
reliable way during Python shutdown.

(cherry picked from commit 5804f878e779712e803be927ca8a6df389d82cdf)
(cherry picked from commit e97c8b0688bc62959ced477d842fcd37992ef649)