Explicitly pin needed versions for Python2.

Status Job ID Name Coverage
  External
failed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

failed SlapOS.SoftwareReleases.IntegrationTest-Master

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch