Python: Make IterableSequenceNode LocalSourceNode

We do this to remove the inconsistencies, and to be ready for a future
where type-tracking support content tracker of depth > 1.

It works because targets of loadSteps needs to be LocalSourceNodes

predicate loadStep(Node nodeFrom, LocalSourceNode nodeTo, Content content) {

Author: RasmusWL

python/ql/lib/semmle/python/dataflow/new/internal/LocalSources.qll

@@ -74,6 +74,8 @@ class LocalSourceNode extends Node {
     this instanceof ScopeEntryDefinitionNode
     or
     this instanceof ParameterNode
+    or
+    this instanceof IterableSequenceNode
   }
 
   /** Holds if this `LocalSourceNode` can flow to `nodeTo` in one or more local flow steps. */