Re: [PATCH] ci: fix Python dependency on Ubuntu 24.04

> Newer versions of Ubuntu have dropped Python 2 starting with Ubuntu
> 23.04. By default though, our CI setups will try to use that Python
> version on all Ubuntu-based jobs except for the "linux-gcc" one.
> 
> We didn't notice this issue due to two reasons:
> 
>   - The "ubuntu:latest" tag always points to the latest LTS release.
>     Until a few weeks ago this was Ubuntu 22.04, which still had Python
>     2.
> 
>   - Our Docker-based CI jobs had their own script to install
>     dependencies until 9cdeb34b96 (ci: merge scripts which install
>     dependencies, 2024-04-12), where we didn't even try to install
>     Python at all for many of them.
> 
> Since the CI refactorings have originally been implemented, Ubuntu
> 24.04 was released, and it being an LTS versions means that the "latest"
> tag now points to that Python-2-less version. Consequently, those jobs
> that use "ubuntu:latest" broke.
> 
> Address this by using Python 2 on Ubuntu 20.04, only, whereas we use
> Python 3 on all other Ubuntu jobs. Eventually, we should think about
> dropping support for Python 2 completely.
> 
> Reported-by: Justin Tobler <jtobler@gmail.com>