Update ENV_VERSION for #13151

#13151 changed the Environment class, but the `ENV_VERSION` has not been updated. This causes errors when building with existing environments.

sphinx/environment/__init__.py

@@ -71,7 +71,7 @@
 
 # This is increased every time an environment attribute is added
 # or changed to properly invalidate pickle files.
-ENV_VERSION = 64
+ENV_VERSION = 65
 
 # config status
 CONFIG_UNSET = -1