fix: install python3 if it is not present

Presentation

Applies the same type of fix for python3 as was previously done for awk.

Fixes #158 (closed)

Checklist

• Documented:
  • README.md reflects any job, variable or whichever visible change
  • kicker.json reflects any job, variable or whichever visible change
• Tested & examplified:
  • (url to a project sample successfully proving the merge request fixes the issue)