BUG : removed unicode string marker for backend_qt4

 - no longer needed due to import string literal
 - close matplotlib#3028

Author: tacaswell

lib/matplotlib/backends/backend_qt4.py

@@ -371,7 +371,7 @@ def _get_key(self, event):
                 key = key.lower()
 
         mods.reverse()
-        return u'+'.join(mods + [key])
+        return '+'.join(mods + [key])
 
     def new_timer(self, *args, **kwargs):
         """