Installing Python 3.9

Hi,

I would like to use software which requires Python 3.9 (it is "installed" via pip). For some reason (something to do with Qt and systemd) the packaged version of this software is wildly out of date on Debian (and I assume most other distros that don't have user-packaged packages e.g. Arch User Repository).

I am using Trisquel 10.0 LTS Nabia, variant Triskel (KDE as my desktop). My python3 package displays (in Synaptic Package Manager) 3.8.2-0ubuntu2 for both the "Installed Version" and "Latest Available Version".

I didn't find much useful information on the Python websites (for Linux they seem to simply direct you to building from source) nor a quick internet search, but [this answer on Unix & Linux Stack Exchange](https://unix.stackexchange.com/a/310846) inspired me to search in Synaptic for python3.9. Sure enough, it exists. However I got stuck on the next step of my process for installing this software: installing python3-venv.

user@hostname:~$ sudo apt-get install python3-venv
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
python3-venv : Depends: python3.8-venv (>= 3.8.2-1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

That is the output after I had used Synaptic to remove python3.9 and then ran sudo apt-get autoremove. An internet search didn't help with this, because I ended up on [Ask Ubuntu where nobody seemed to know what they were doing](https://askubuntu.com/questions/564282/apt-get-unmet-dependencies-but-it-is-not-going-to-be-installed). I've encountered this issue a handful of times before, and have usually solved it by flailing my commands around similar to the answers on Ask Ubuntu. But I hope someone on these forums may be more knowledgeable.

P.S. It's unlikely that anyone here is a maintainer/designer/otherwise involved in package managers, but what an awful error message "but it is not doing to be installed". I assume, like most things, the system is more complex than I could ever imagine. I still struggle to fathom that more information can't be provided.