software/slapos-testing: Fix Python2-incompatible versions

statsmodels and websocket-client were pinned to Python3-only versions,
and slapos.toolbox was installed in develop mode from a Python3-only
revision.
18 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:36:04

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:01:50

failed SlapOS.Eggs.UnitTest-Master.Python2

failed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.SoftwareReleases.IntegrationTest-Master

12:24:32

passed SlapOS.Theia.ResilienceTest-Master-ERP5

08:49:06

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:36:55

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

11:59:50

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:31:23

failed SlapOS.Eggs.UnitTest-Master.Python2

03:02:36

failed SlapOS.Eggs.UnitTest-Master.Python2

00:17:14

failed SlapOS.Eggs.UnitTest-Master.Python2

03:02:55

failed SlapOS.Eggs.UnitTest-Master.Python3

00:23:28

failed SlapOS.Eggs.UnitTest-Master.Python3

00:15:14

failed SlapOS.Eggs.UnitTest-Master.Python3

00:15:18

failed SlapOS.SoftwareReleases.IntegrationTest-Master

08:00:41

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:33:23