fslint on Kub 20.04?

[Ralf Mardorf]

Don't edit the apt source list. Download the packages with wget or curl
and install them using apt. Maybe "apt install URL" works, too.

However, changing the mirror gains nothing. The versions that work for
other aren't from any still supported Ubuntu release, let alone that
edited repositories are way more dangerous than installing the packages
from a local path.

http://mirror.ip-projects.de/ubuntu/pool/universe/p/pygtk/python-gtk2_2.24.0-6_amd64.deb
http://mirror.ip-projects.de/ubuntu/pool/universe/p/pygtk/python-glade2_2.24.0-6_amd64.deb
...

Or see
https://www.codegrepper.com/code-examples/shell/Package+%27fslint%27+has+no+installation+candidate
for http://old-releases.ubuntu.com/ubuntu/pool/universe/p/pygtk/
...

Can of worms:
https://towardsdatascience.com/installing-multiple-alternative-versions-of-python-on-ubuntu-20-04-237be5177474
https://wiki.archlinux.org/title/python

You probably want to check the packages against a signed checksum, but
doing this opens another can of worms.

TL;DR
https://blog.packagecloud.io/eng/2014/10/28/howto-gpg-sign-verify-deb-packages-apt-repositories/
You probably only can verify the source. IOW if the source is ok, you
need to build the package.

Assuming there's no issue with installing the packages and no issue
using multiple versions of python, you still need to ensure that the
packages you want to install were not manipulated. If they aren't
manipulated, then you need still to check the software versions against
Common Vulnerabilities and Exposures.

https://cve.mitre.org/find/search_tips.html