software/slapos-testing: stop testing slapos.toolbox on python2

This only support python3 since some time
6 jobs for master in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:06:06

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:02:14

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

00:32:05

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:07

failed SlapOS.SoftwareReleases.IntegrationTest-Master

06:04:29

passed SlapOS.Theia.ResilienceTest-Master-ERP5

07:37:04