Replace python2 with python3

What does this merge request do and why?

This MR replaces python2 with python3 in the packages_arch.txt as it's no longer available, at least not in the archlinux:base Docker image:

$ docker run --rm -ti archlinux:base pacman -Sy python2
:: Synchronizing package databases...
 core downloading...
 extra downloading...
 community downloading...
error: target not found: python2

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.