Validate the authMechanism URI option.

def validate_auth_mechanism(option, value):
    """Validate the authMechanism URI option.
    """
    # CRAM-MD5 is for server testing only. Undocumented,
    # unsupported, may be removed at any time. You have
    # been warned.
    if value not in MECHANISMS and value != 'CRAM-MD5':
        raise ValueError("%s must be in %s" % (option, tuple(MECHANISMS)))
    return value