[#4] Demo durus import causing worker to hang

This commit demonstrates the worker process hanging due to the addition
of the following import:

    from durus.persistent_list import PersistentList

Note that the behaviour also happens with any of the following import
statements:

    import durus.persistent_list
    import durus.persistent_dict
    from durus.persistent_dict import PersistentDict

See #4.