Python: Fix points-to for a, b = some_unknown_object.

Also rewrites `multi_assignment_points_to` to be a bit more readable.

I'm not entirely sure that we want an unknown instance of `object` rather than
just `UnknownInternal`. The latter gets filtered out in the characteristic
predicate for `Value`, though, so I opted for the slightly more permissive
variant.

Author: tausbn

python/ql/src/semmle/python/pointsto/PointsTo.qll

@@ -506,13 +506,21 @@ cached module PointsToInternal {
         value = sequence.getItem(index)
     }
 
-    pragma [noinline]
-    private predicate multi_assignment_points_to(MultiAssignmentDefinition def, PointsToContext context, ObjectInternal value, ControlFlowNode origin) {
-        exists(int index, ControlFlowNode lhs, ControlFlowNode rhs, SequenceObjectInternal sequence |
+    pragma[noinline]
+    private predicate multi_assignment_points_to(
+        MultiAssignmentDefinition def, PointsToContext context, ObjectInternal value,
+        ControlFlowNode origin
+    ) {
+        exists(int index, ControlFlowNode lhs, ControlFlowNode rhs, ObjectInternal sequence |
             def.indexOf(index, lhs) and
             lhs.(DefinitionNode).getValue() = rhs and
-            sequence_index_points_to(rhs, context, sequence, value, index) and
             origin = def.getDefiningNode()
+        |
+            sequence_index_points_to(rhs, context, sequence, value, index)
+            or
+            pointsTo(rhs, context, sequence, _) and
+            sequence instanceof UnknownInstanceInternal and
+            value = TUnknownInstance(ObjectInternal::builtin("object"))
         )
     }