[3.6] bpo-30807: signal.setitimer() may disable the timer by mistake (GH-2493) (#2497)

* bpo-30807: signal.setitimer() may disable the timer by mistake

* Add NEWS blurb
(cherry picked from commit 729780a810bbcb12b245a1b652302a601fc9f6fd)
diff --git a/Lib/test/test_signal.py b/Lib/test/test_signal.py
index ab42ed7..aa2b8bb 100644
--- a/Lib/test/test_signal.py
+++ b/Lib/test/test_signal.py
@@ -611,6 +611,15 @@
         # and the handler should have been called
         self.assertEqual(self.hndl_called, True)
 
+    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)
+
 
 class PendingSignalsTests(unittest.TestCase):
     """