> Without _stopped, join() can simply wait to acquire _tstate_lock (with
> or without a timeout, and skipping this if _tstate_lock is already
> None).  Etc ;-)  Of course details matter, but it's easy.  I did it
> once, but the tests joining the main thread failed and I put the code
> on hold.

Ah, of course. The main thread needs the event, since the thread state
will only be deleted at the end of Py_Finalize().
The MainThread class could override is_alive() and join(), then.