Fix format string for python < 3

theano/configdefaults.py

@@ -1679,4 +1679,4 @@ AddConfigVar(
 
 # Check if there are remaining flags provided by the user through THEANO_FLAGS.
 for key in THEANO_FLAGS_DICT.keys():
-    warnings.warn('Theano does not recognise this flag: {}'.format(key))
+    warnings.warn('Theano does not recognise this flag: {0}'.format(key))