component/slapos: fix slapos-node package build after python3.8

This is a fixup of bddc5491.
11 jobs for django-tutorial in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

15:03:12

failed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

15:03:14

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

15:03:11

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

15:02:54

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:03:09

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

failed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch