Python: Use only TApiNode in API::Impl

This ensures that changes to `API::Node` does not invalidate the cached
`module Impl`. At present, I don't expect this to have any effect (as
the `Node` class is also fairly static, though not explicitly cached),
but I can imagine us making some of the `Node` methods have
user-extensible behaviour, in which case we definitely do not want this
to result in reevaluation of `API::Impl`.

Author: tausbn

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

@@ -452,7 +452,7 @@ module API {
      * Holds if there is an edge from `pred` to `succ` in the API graph that is labeled with `lbl`.
      */
     cached
-    predicate edge(Node pred, string lbl, Node succ) {
+    predicate edge(TApiNode pred, string lbl, TApiNode succ) {
       /* There's an edge from the root node for each imported module. */
       exists(string m |
         pred = MkRoot() and