Oh, in fact, it's not that hard to reproduce the failure on Linux:

haypo@selma$ ./python -m test test_pty -m test_basic -F 
Run tests sequentially
0:00:00 load avg: 2.17 [  1] test_pty
0:00:00 load avg: 2.17 [  2] test_pty
0:00:00 load avg: 2.17 [  3] test_pty
(...)
0:00:00 load avg: 2.17 [ 15] test_pty
0:00:00 load avg: 2.17 [ 16] test_pty
test test_pty failed -- Traceback (most recent call last):
  File "/home/haypo/prog/python/master/Lib/test/test_pty.py", line 105, in test_basic
    self.assertEqual(b'For my pet fish, Eric.\n', normalize_output(s2))
AssertionError: b'For my pet fish, Eric.\n' != b'For m'

test_pty failed
15 tests OK.

1 test failed:
    test_pty

Total duration: 486 ms
Tests result: FAILURE