These are scripts that create "alternatives" for use by the Debian
update-alternatives program.
The alternatives are "link groups" that get installed as a bundle when a given master file is selected.
This particular set is for python3.
After installing, you can run update-alternatives --set python3 SOMETHING
and choose which alternative you want to serve as your python3 package.
You must have root via su, or be able to use sudo, in order to run
these scripts.
If you run as root, the scripts run update-alternatives directly. If
you run as not-root, the scripts run sudo update-alternatives.
If you can't become root, and don't have sudo access, then you can't install
alternatives. Try the pythonz package for managing multiple python versions
in your home directory, instead.