Fix minor code style

Change-Id: Ib051581cd820cbb478b95b1b4421adb2b577567d

taskflow/backends/memory.py

@@ -154,9 +154,7 @@ class MemoryLogBook(logbook.LogBook):
 
     @check_not_closed
     def __contains__(self, flow_name):
-        if flow_name not in self._flow_names:
-            return False
-        return True
+        return flow_name in self._flow_names
 
     def __delitem__(self, flow_name):
         w = self[flow_name]

taskflow/patterns/base.py

@@ -59,7 +59,7 @@ class Flow(object):
         if parents:
             self.parents = tuple(parents)
         else:
-            self.parents = tuple([])
+            self.parents = ()
         # Any objects that want to listen when a wf/task starts/stops/completes
         # or errors should be registered here. This can be used to monitor
         # progress and record tasks finishing (so that it becomes possible to