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

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.

Include/cpython/pystate.h

@@ -55,6 +55,7 @@ struct _ts {
     struct _ts *next;
     PyInterpreterState *interp;
 
+    /* Borrowed reference to the current frame (it can be NULL) */
     struct _frame *frame;
     int recursion_depth;
     char overflowed; /* The stack has overflowed. Allow 50 more calls