> FWIW, Issue29589 seems to be an older example of this failure.

It was a failure on Mageia 5 (Linux) at 2017-02-17:

== CPython 3.6.0 (default, Feb 17 2017, 15:26:31) [GCC 4.9.2]
==   Linux-4.4.13-desktop-1.mga5-x86_64-with-mageia-5-Official little-endian
==   hash algorithm: siphash24 64bit
==  cwd: /home/dima/bin/Python-3.6.0/build/test_python_8244
==  encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 [1/1] test_multiprocessing_forkserver
...
======================================================================
ERROR: test_ignore (test.test_multiprocessing_forkserver.TestIgnoreEINTR)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dima/bin/Python-3.6.0/Lib/test/_test_multiprocessing.py", line 3728, in test_ignore
    os.kill(p.pid, signal.SIGUSR1)
ProcessLookupError: [Errno 3] No such process