It's related to bpo-30807, "setitimer() can disable timer by mistake".

Test code:

    def test_setitimer_tiny(self):
        # bpo-30807: C setitimer() takes a microsecond-resolution interval.
        # Check that float -> timeval conversion doesn't round
        # the interval down to zero, which would disable the timer.
        self.itimer = signal.ITIMER_REAL
        signal.setitimer(self.itimer, 1e-6)
        time.sleep(1)
        self.assertEqual(self.hndl_called, True)