stack/slapos: version down zc.buildout 2.7.1+slapos016 slapos.libnetworkcache 0.22 on python3

This partially reverts ee733f9d because
of issues with python3
6 jobs for django-tutorial
Status Job ID Name Coverage
  External
failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:44:18

passed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

08:26:11

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

08:12:09

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5