Python: Handle class attribute references in API graph

This is slightly dubious, and should really be in the currently
unimplemented "def" counterpart to the "use" bits we already have.

However, it seems to work correctly, and in the spirit of moving
things along, this seemed like the easier solution. We can always
replace the implementation with the "proper" approach at a later point.

python/ql/src/semmle/python/ApiGraphs.qll

@@ -356,9 +356,9 @@ module API {
         // the relationship between `pred` and `ref`.
         use(base, src) and pred = trackUseNode(src)
       |
-        // Reading an attribute on a node that is a use of `base`:
+        // Referring to an attribute on a node that is a use of `base`:
         lbl = Label::memberFromRef(ref) and
-        ref = pred.getAnAttributeRead()
+        ref = pred.getAnAttributeReference()
         or
         // Calling a node that is a use of `base`
         lbl = Label::return() and