exekall: Make serialization test more robust
pickle.dumps() seems to also raise AttributeError sometimes, in addition to pickle.PickleError: AttributeError: Can't pickle local object 'SpawnBase.init..write_to_stdout
pickle.dumps() seems to also raise AttributeError sometimes, in addition to pickle.PickleError: AttributeError: Can't pickle local object 'SpawnBase.init..write_to_stdout