add index to format to support python2.6 (#5104)

IPython/html/base/handlers.py

@@ -122,7 +122,7 b' class AuthenticatedHandler(RequestHandler):'
 
     @property
     def cookie_name(self):
-        default_cookie_name = non_alphanum.sub('-', 'username-{}'.format(
+        default_cookie_name = non_alphanum.sub('-', 'username-{0}'.format(
             self.request.host
         ))
         return self.settings.get('cookie_name', default_cookie_name)