Prompt for an MFA serial number

    :param input:                       Used for unit testing

    :return:                            The MFA serial number

def prompt_4_mfa_serial(input = None):
    """
    Prompt for an MFA serial number

    :param input:                       Used for unit testing

    :return:                            The MFA serial number
    """
    return prompt_4_value('Enter your MFA serial:', required = False, regex = re_mfa_serial_format, regex_format = mfa_serial_format, input = input)