Use python3 for caucase, jscrawler and htmlvalidatorserver softwares

See merge request !1243
3 jobs for feat/slapos-sr-testing-py3 in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:38:51

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

04:44:18

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3