python: move model to Stdlib.yml

There is already a model there so we add to that one.

We did observe that this existing model was blocked by the external MaD model.
This is concerning and needs to be cleared up.

python/ql/lib/semmle/python/frameworks/Stdlib.qll

@@ -3251,8 +3251,13 @@ module StdlibPrivate {
 
     override predicate propagatesFlow(string input, string output, boolean preservesValue) {
       input in ["Argument[0]", "Argument[pattern:]"] and
-      output = "ReturnValue.Attribute[pattern]" and
-      preservesValue = true
+      (
+        output = "ReturnValue.Attribute[pattern]" and
+        preservesValue = true
+        or
+        output = "ReturnValue" and
+        preservesValue = false
+      )
     }
   }