Get the machine portion of an ObjectId.

def _machine_bytes():
    """Get the machine portion of an ObjectId.
    """
    machine_hash = hashlib.md5()
    if PY3:
        # gethostname() returns a unicode string in python 3.x
        # while update() requires a byte string.
        machine_hash.update(socket.gethostname().encode())
    else:
        # Calling encode() here will fail with non-ascii hostnames
        machine_hash.update(socket.gethostname())
    return machine_hash.digest()[0:3]