Return True: The client is re-registered to the selector object.
    Return False: The server disconnects the client.

def handle_readable(client):
    """
    Return True: The client is re-registered to the selector object.
    Return False: The server disconnects the client.
    """

    data = client.recv(1028)
    if data == b'':
        return False
    client.sendall(b'SERVER: ' + data)
    print(threading.active_count())
    return True