Keep Python2 compatibility for the documentation

readthedocs.org build with a Python2 environment.

doc/conf.py

@@ -13,6 +13,7 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 #
+from __future__ import unicode_literals
 # import sys, os
 
 # If extensions (or modules to document with autodoc) are in another directory,