As discussed in issue 18808, now that we're checking for a tstate lock, the Thread._stopped Event has become an "attractive nuisance".  The attached patch removes it.

This simplifies .join() and .is_alive(), and restores pre-18808 .join(timeout) endcase behavior (i.e., if join returns before the timeout expires, .is_alive() will always be False).

Since this doesn't add any new locks, I hope it won't create more problems with fork tests - but running on Windows I wouldn't know ;-)