Keep Python2 compatibility for the documentation

readthedocs.org build with a Python2 environment.

doc/conf.py

@@ -12,6 +12,7 @@
 #
 # All configuration values have a default; values that are commented out
 # serve to show the default.
+from __future__ import unicode_literals
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the