Remove python2 specific code

IPython/core/history.py

@@ -514,9 +514,6 b' class HistoryManager(HistoryAccessor):'
     # History saving in separate thread
     save_thread = Instance('IPython.core.history.HistorySavingThread',
                            allow_none=True)
-    try:               # Event is a function returning an instance of _Event...
-        save_flag = Instance(threading._Event, allow_none=True)
-    except AttributeError:         # ...until Python 3.3, when it's a class.
         save_flag = Instance(threading.Event, allow_none=True)
     
     # Private interface