vantage6's Pickle serialization is insecure

vantage6 is privacy preserving federated learning infrastructure. Versions prior to 4.0.0 use pickle, which has known security issue, as a default serialization module but that has known security issues. All users of vantage6 that post tasks with the default serialization are affected. Version 4.0.0 contains a patch. Users may specify JSON serialization as a workaround.