Closes #1070: Avoid un-pickling issues under Python 3 when accessing doctrees saved under Python 2.

CHANGES

@@ -1,6 +1,9 @@
 Release 1.2 (in development)
 ============================
 
+* #1070: Avoid un-pickling issues when running Python 3 and the saved
+  environment was created under Python 2.
+
 * #1069: Fixed error caused when autodoc would try to format signatures of
   "partial" functions without keyword arguments (patch by Artur Gaspar).
 

sphinx/environment.py

@@ -71,7 +71,7 @@ default_settings = {
 
 # This is increased every time an environment attribute is added
 # or changed to properly invalidate pickle files.
-ENV_VERSION = 42
+ENV_VERSION = 42 + (sys.version_info[0] - 2)
 
 
 default_substitutions = set([