I closed bpo-38448 as duplicate of this issue. Copy of its only message:

AMD64 RHEL8 Refleaks 3.x:
https://buildbot.python.org/all/#/builders/272/builds/23

0:27:13 load avg: 4.88 [416/419/1] test_concurrent_futures failed (env changed) (17 min 11 sec) -- running: test_capi (7 min 28 sec), test_gdb (8 min 49 sec), test_asyncio (23 min 23 sec)
beginning 6 repetitions
123456
.Warning -- reap_children() reaped child process 26487
.....
Warning -- multiprocessing.process._dangling was modified by test_concurrent_futures
  Before: set()
  After:  {<weakref at 0x7fdc08f44e30; to 'SpawnProcess' at 0x7fdc0a467c30>}