Python3: Cleanup nuitka3 script to get Python3 version list from package.
Python3: Cleanup nuitka3 script to get Python3 version list from package.

* This uses "getSupportedPythonVersions" to avoid having a hard coded
  list of Python versions here.

* Also use proper "exec" instead of merely forking, which should be more
  efficient and propagate error exit code cleanly.

kayhayen/Nuitka,kayhayen/Nuitka,kayhayen/Nuitka,kayhayen/Nuitka