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/call.ql

@@ -7,6 +7,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from Value len, CallNode call
 where len.getName() = "len" and len.getACall() = call

python/ql/examples/snippets/extend_class.ql

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

python/ql/examples/snippets/method_call.ql

@@ -7,6 +7,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from AstNode call, PythonFunctionValue method
 where

python/ql/examples/snippets/mutualrecursion.ql

@@ -7,6 +7,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from FunctionObject m, FunctionObject n
 where m != n and m.getACallee() = n and n.getACallee() = m

python/ql/examples/snippets/override_method.ql

@@ -7,6 +7,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from FunctionObject override, FunctionObject base
 where

python/ql/examples/snippets/recursion.ql

@@ -7,6 +7,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from PythonFunctionValue f
 where f.getACall().getScope() = f.getScope()