Marked possibly failing test with a TODO note.

celery/tests/test_task.py

@@ -475,6 +475,7 @@ class TestPeriodicTask(unittest.TestCase):
     def test_is_due_not_due(self):
         due, remaining = MyPeriodic().is_due(datetime.now())
         self.assertFalse(due)
+        # TODO: This assertion may fail if executed in the first minute of an hour
         self.assertGreater(remaining, 60)
 
     def test_is_due(self):