To be more confident that the stream handler is registered in the daemon.
- Use `from_id`, the changed field name in `PSMessage`. - PR feedbacks - Add label `test` in `testenv` in tox.ini, to avoid wrong dispatching an environment's command in the future. - Use `pytest` over `py.test`.
It is moved to the top level package `tests_interop`, to avoid circular dependency, with the dependency moved to `tox`.