test_selectors: test_timeout fails sometimes on busy (slow) buildbots, relax

test_selectors: test_timeout fails sometimes on busy (slow) buildbots, relax
the unit test on max time (but be more strict on mon time). Example of failure:

http://buildbot.python.org/all/builders/x86%20OpenIndiana%203.x/builds/6978/steps/test/logs/stdio

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FAIL: test_timeout (test.test_selectors.PollSelectorTestCase)
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Traceback (most recent call last):
  File "/export/home/buildbot/32bits/3.x.cea-indiana-x86/build/Lib/test/test_selectors.py", line 316, in test_timeout
    self.assertTrue(0.5 < t1 - t0 < 1.5, t1 - t0)
AssertionError: False is not true : 1.5033390671014786