gress_callback can be set to a function(sent, total) which is regularly
    called with the number of bytes already sent and total number of bytes to
    send. It is called every 0.5 to 2 seconds (dynamically adapted to upload
    bandwidth).

    Return None on error, or the ticket number on success.

    By default this uses the production Launchpad hostname. Set
    hostname to 'launchpad.dev' or 'staging.launchpad.net' to use another
    instance for testing.
    Nz