Under python 3 we must use bytes.

Change-Id: I86d2a875f4e06a9cb9724d86348f420bb8ea19e9
Reviewed-on: https://gerrit.libreoffice.org/25084Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>