Clean up serialize_ast example to use HIGHEST_PROTOCOL for pickling

examples/serialize_ast.py

@@ -25,13 +25,8 @@
     parser = c_parser.CParser()
     ast = parser.parse(text)
 
-    # Since AST nodes use __slots__ for faster attribute access and
-    # space saving, it needs Pickle's protocol version >= 2.
-    # The default version is 3 for python 3.x and 1 for python 2.7.
-    # You can always select the highest available protocol with the -1 argument.
-
     with open('ast', 'wb') as f:
-        pickle.dump(ast, f, protocol=-1)
+        pickle.dump(ast, f, protocol=pickle.HIGHEST_PROTOCOL)
 
     # Deserialize.
     with open('ast', 'rb') as f: