test_semaphore_tracker_sigint() emits a warning. If the test is run with -W error, the test fails.

vstinner@apu$ ./python -m test test_multiprocessing_fork -v -m test_semaphore_tracker_sigint
(...)
test_semaphore_tracker_sigint (test.test_multiprocessing_fork.TestSemaphoreTracker) ... /home/vstinner/prog/python/master/Lib/multiprocessing/semaphore_tracker.py:55: UserWarning: semaphore_tracker: process died unexpectedly, relaunching.  Some semaphores might leak.
  warnings.warn('semaphore_tracker: process died unexpectedly, '
ok
(...)
Tests result: SUCCESS

vstinner@apu$ ./python  -Werror  -m test test_multiprocessing_fork -v -m test_semaphore_tracker_sigint
(...)
======================================================================
ERROR: test_semaphore_tracker_sigint (test.test_multiprocessing_fork.TestSemaphoreTracker)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/_test_multiprocessing.py", line 4533, in test_semaphore_tracker_sigint
    self.check_semaphore_tracker_death(signal.SIGINT, False)
  File "/home/vstinner/prog/python/master/Lib/test/_test_multiprocessing.py", line 4521, in check_semaphore_tracker_death
    sem = ctx.Semaphore()
  File "/home/vstinner/prog/python/master/Lib/multiprocessing/context.py", line 82, in Semaphore
    return Semaphore(value, ctx=self.get_context())
  File "/home/vstinner/prog/python/master/Lib/multiprocessing/synchronize.py", line 127, in __init__
    SemLock.__init__(self, SEMAPHORE, value, SEM_VALUE_MAX, ctx=ctx)
  File "/home/vstinner/prog/python/master/Lib/multiprocessing/synchronize.py", line 81, in __init__
    register(self._semlock.name)
  File "/home/vstinner/prog/python/master/Lib/multiprocessing/semaphore_tracker.py", line 83, in register
    self._send('REGISTER', name)
  File "/home/vstinner/prog/python/master/Lib/multiprocessing/semaphore_tracker.py", line 90, in _send
    self.ensure_running()
  File "/home/vstinner/prog/python/master/Lib/multiprocessing/semaphore_tracker.py", line 55, in ensure_running
    warnings.warn('semaphore_tracker: process died unexpectedly, '
UserWarning: semaphore_tracker: process died unexpectedly, relaunching.  Some semaphores might leak.
(...)
Tests result: FAILURE