Fix #5361: crashed on incremental build if document uses include directive

CHANGES

@@ -18,6 +18,8 @@ Features added
 Bugs fixed
 ----------
 
+* #5361: crashed on incremental build if document uses include directive
+
 Testing
 --------
 

sphinx/environment/__init__.py

@@ -72,7 +72,7 @@ default_settings = {
 # or changed to properly invalidate pickle files.
 #
 # NOTE: increase base version by 2 to have distinct numbers for Py2 and 3
-ENV_VERSION = 52 + (sys.version_info[0] - 2)
+ENV_VERSION = 54 + (sys.version_info[0] - 2)
 
 
 versioning_conditions = {