Oh. test_daemon_threads_shutdown_stdout_deadlock of test_io fails randomly on buildbots.

I ran "./python -m test test_io -m '*daemon*' -F -j4" for 5 minutes on my Fedora 29 before merging my PR, but I didn't get any failure :-(

======================================================================
FAIL: test_daemon_threads_shutdown_stdout_deadlock (test.test_io.CMiscIOTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel-z/build/Lib/test/test_io.py", line 4186, in test_daemon_threads_shutdown_stdout_deadlock
    self.check_daemon_threads_shutdown_deadlock('stdout')
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel-z/build/Lib/test/test_io.py", line 4177, in check_daemon_threads_shutdown_deadlock
    self.assertIn("Fatal Python error: could not acquire lock "
AssertionError: "Fatal Python error: could not acquire lock for <_io.BufferedWriter name='<stdout>'> at interpreter shutdown, possibly due to daemon threads" not found in 'Fatal Python error: PyCOND_FINI(_PyRuntime.ceval.gil.cond) failed'

Failures:

AMD64 Fedora Rawhide Clang Installed 3.x: fail then pass
https://buildbot.python.org/all/#/builders/188/builds/93

s390x Debian 3.x: fail then pass
https://buildbot.python.org/all/#/builders/13/builds/2799

s390x RHEL 3.x: fail twice
https://buildbot.python.org/all/#/builders/21/builds/2780

AMD64 Debian root 3.x: fail twice
https://buildbot.python.org/all/#/builders/27/builds/2759