Update Python bindings example to Python 3 syntax (#978)

Author: david-poirier

doc/ref/bindings.html

@@ -173,11 +173,11 @@ <h2 id="python_api">Python API</h2>
     ext_vars={'OTHER_NAME': 'Bob'})
 
 json_obj = json.loads(json_str)
-for person_id, person in json_obj.iteritems():
-  print '%s is %s, greeted by "%s"' % (
+for person_id, person in json_obj.items():
+  print('%s is %s, greeted by "%s"' % (
       person_id,
       person['name'],
-      person['welcome'])</pre>
+      person['welcome']))</pre>
     <div style="clear: both"></div>
   </div>
 </div>