I see the same problem.   I'm not sure my code is clean,
but I'll be darned if I can find the problem.

I've traced my code with print statements,
and I see some threads reach their return statement,
but the process hangs when you try to join() that thread.
It seems that there is something in the clean-up code
in threading.py (presumably) that can hang under some
obscure circumstances.

As a result, these threads don't terminate on time,
and various exceptions happen as python is being
dismantled and all the variables de-allocated.