test_urllib failed and then passed when re-run on s390x RHEL7 Refleaks 3.x:
https://buildbot.python.org/all/#builders/129/builds/300

I can reproduce the issue on my Linux laptop:

$ ./python -m test -m unittest.test.test_break.TestBreakDefaultIntHandler.testInterruptCaught test_unittest -F
0:00:00 load avg: 1.52 Run tests sequentially
0:00:00 load avg: 1.52 [  1] test_unittest
0:00:00 load avg: 1.52 [  2] test_unittest
0:00:00 load avg: 1.52 [  3] test_unittest
0:00:00 load avg: 1.52 [  4] test_unittest
0:00:00 load avg: 1.52 [  5] test_unittest
0:00:01 load avg: 1.52 [  6] test_unittest
0:00:01 load avg: 1.52 [  7] test_unittest
0:00:01 load avg: 1.52 [  8] test_unittest
test test_unittest failed -- Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/unittest/test/test_break.py", line 66, in testInterruptCaught
    test(result)
    ^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/unittest/test/test_break.py", line 63, in test
    self.assertTrue(result.shouldStop)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: False is not true

test_unittest failed (1 failure)

== Tests result: FAILURE ==

7 tests OK.

1 test failed:
    test_unittest

Total duration: 1.7 sec
Tests result: FAILURE