Python: Fix example snippets

One might argue that these should be rewritten entirely to use more
modern APIs, but for now I'll be content with just having them compile
properly.

python/ql/examples/snippets/extend_class.ql

@@ -11,6 +11,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from ClassObject sub, ClassObject base
 where