Hello,
I think might be seeing a [bug in python 3.5 weakref]
(Issue 29519: weakref spewing exceptions during finalization when combined with multiprocessing - Python tracker) that occurs during shutdown.
On my machine I was able to resolve it by applying this patch (though I seem to recall that there was some fuzz in the line numbers):
https://github.com/python/cpython/commit/9cd7e17640a49635d1c1f8c2989578a8fc2c1de6.patch
Best regards
Thomas