Python: Improve CallCfgNode interface

Call nodes are always local sources (specifically sources of the return
value of the call), and so inheriting from `LocalSourceNode` will have
no effect on results, but _should_ make it a bit more smooth to use the
API.

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -180,7 +180,7 @@ class CfgNode extends Node, TCfgNode {
 }
 
 /** A data-flow node corresponding to a `CallNode` in the control-flow graph. */
-class CallCfgNode extends CfgNode {
+class CallCfgNode extends CfgNode, LocalSourceNode {
   override CallNode node;
 
   /**