So:

#1 we've a bug: the single-threaded ProcessPoolExecutor test program should work 100% reliably - it does not

#2 we've a cause: ProcessPoolExecutor is implemented internally using an unfortunate combination of fork and threads, this is causing the deadlock

#3 we've got a workaround - something like:
   ProcessPoolExecutor(multiprocessing.get_context('spawn'))
but I'm guessing, the documentation is scant.

As for a fix, maybe:
- have ProcessPoolExecutor use 'spawn' by default; this way things always work
- have ProcessPoolExecutor properly synchronized its threads before "spawning"/"forking"/... so that "single-threaded" code works
- document that combining ProcessPoolExecutor's "fork" option and user threads isn't a good idea