Fix a bug when throwing an exception with unicode characters from

Weboob.

As reported by @nicofrand.

flatisfy/fetch.py

@@ -3,6 +3,7 @@
 This module contains all the code related to fetching and loading flats lists.
 """
 from __future__ import absolute_import, print_function, unicode_literals
+from builtins import str
 
 import collections
 import itertools