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.
3 jobs for feat/erp5_python3 in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

04:03:48

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

05:24:06

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

21:01:11