slapos_panel: [py2/py3] str.encode('hex') does not work in Python 3.

   Probably this was based on some outdated version, since the calculation already changed on:

       nexedi/erp5@6ded2571
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Rafael

08:47:20

passed SlapOS-Master.UnitTest-Rafael

01:35:18

passed SlapOS-Master.UnitTest-Rafael

01:41:20

passed SlapOS-Master.UnitTest-Rafael

02:07:08