software/slapos-sr-testing: run ERP5 SR test with a python3 slapos.core

Now that "the bug in the way promises are run" referenced in the comment
in ${template:test} is being fixed, we can start running ERP5 with
python3 slapos.
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

04:34:39

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:46:14

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

04:38:03