I don't know for sure that this is the cause but both 3.x builds following this commit on my bolen-ubuntu worker (Ubuntu 18.04.3) have had test_pty crash in the first attempt, with the retry succeeding. For example https://buildbot.python.org/all/#/builders/141/builds/2679
In spot checking, the same behavior seems to be occurring on many other non-Windows builders as well, presumably those for which it gets used.
There doesn't seem to be anything useful produced by the first crash. Just an entry for "test_pty crashed (Exit code -1)" in the test log.