Indicate this code is Python 3 only

Clarify due to #156

README.rst

@@ -149,7 +149,7 @@ If you're using Pywikibot_, your code might look like this::
         text = page.get()
         return mwparserfromhell.parse(text)
 
-If you're not using a library, you can parse any page using the following code
+If you're not using a library, you can parse any page using the following Python 3 code
 (via the API_)::
 
     import json