Python: address review comments

python/ql/test/experimental/dataflow/coverage/classes.py

@@ -1041,7 +1041,9 @@ def test_index():
   with_index = With_index()
   operator.index(with_index)
 
-# make With_index subscriptable to test slicing
+def test_index_slicing():
+  with_index = With_index()
+  [0][with_index:1]
 
 def test_index_bin():
   with_index = With_index()

python/ql/test/experimental/dataflow/coverage/classesCallGraph.ql

@@ -6,4 +6,5 @@ where
   sink.getLocation().getFile().getBaseName() = "classes.py" and
   exists(CallGraphConfig cfg | cfg.hasFlow(source, sink))
 select source, sink
-// Rewrite this to just have 1-step paths?
+// Ideally, we would just have 1-step paths either from argument to parameter
+// or from return to call. This gives a bit more, so should be rewritten.