With either fix, or with both, on Linux I still see this hang at shutdown.  Victor mentioned the fact that he had to hit Ctrl-C on Linux to see this, and I have to do the same thing. Then with the fixes in place the original test case still hangs on shutdown.  

On Python 3.7 (I don't readily have 3.8 available) at least this just runs and completes with no ctrl-C and no crashes.  So while either of the fixes may be good to prevent the crashes, there's still probably some underlying issue in multiprocessing.  I haven't tested on Mac OS/X.

It looks like the clearing was originally introduced here: https://bugs.python.org/issue10241  Interestingly there was a similar issue w/ _tkinter, which also used PyType_FromSpec (although it sounds like it was just a ref count issue on the type).  Unfortunately there's no associated test cases added to verify the behavior.  Antoine and Neil are both now on the PR which removes the collection behavior so hopefully they can chime in on the safety of that fix.