std modules: atexit, json, os, signal, socket, ssl, subprocess, sys, time, threading, xmlrpc.client
The non-standard extension: psutil, PIL (only for image encoding though).

The amount of threads may vary, but i may have around 20 threads at all time. Most of them are indeed demon threads.

I have one atexit handler: i executes a few subprocess and pereform a few xmlrpc calls then exit. In this case, the handler go fully executed.

There are signal handlers, but none of them got called.

No monkeypatching is involved :)

I only browsed the patch up until the 3.5 head. (i guess il lacked to courage to go up to 3.7).

I tried to write a reproduction case, but i failed to run into the error. Of course, i will try to improve it if i get a clue about a way to increase the likelyness of the problem.
Sadly, all i have right now is a process i can attach to.