Python: Re-introduce abstract toString

This seems like the easier solution in the short run.

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

@@ -157,7 +157,7 @@ module API {
     /**
      * Gets a textual representation of this element.
      */
-    string toString() { none() }
+    abstract string toString();
 
     /**
      * Gets a path of the given `length` from the root to this node.

python/ql/src/semmle/python/frameworks/PEP249.qll

@@ -20,7 +20,10 @@ abstract deprecated class PEP249Module extends DataFlow::Node { }
  * An abstract class encompassing API graph nodes that implement PEP 249.
  * Extend this class for implementations.
  */
-abstract class PEP249ModuleApiNode extends API::Node { }
+abstract class PEP249ModuleApiNode extends API::Node {
+  /** Gets a string representation of this element. */
+  override string toString() { result = this.(API::Node).toString() }
+}
 
 /** Gets a reference to a connect call. */
 DataFlow::Node connect() { result = any(PEP249ModuleApiNode a).getMember("connect").getAUse() }