sql: force visiting_objects to be list (#36515)

wcs/sql.py

@@ -1894,7 +1894,7 @@ class Session(SqlMixin, wcs.sessions.BasicSession):
             # the other fields are stored to run optimized SELECT() against the
             # table, they are ignored when loading the data.
             'name_identifier': self.name_identifier,
-            'visiting_objects_keys': self.visiting_objects.keys() if getattr(self, 'visiting_objects') else None,
+            'visiting_objects_keys': list(self.visiting_objects.keys()) if getattr(self, 'visiting_objects') else None,
             'last_update_time': datetime.datetime.now(),
         }