Fixed a new pep8 error and a small typo

Change-Id: I03416abe1c2207265ddd50d49b77a90e4cf5c48b

openstack/common/timeutils.py

@@ -134,7 +134,7 @@ def set_time_override(override_time=None):
 
 def advance_time_delta(timedelta):
     """Advance overridden time using a datetime.timedelta."""
-    assert(not utcnow.override_time is None)
+    assert utcnow.override_time is not None
     try:
         for dt in utcnow.override_time:
             dt += timedelta