Sorry for the late response, but I put this Python problem in pause for quite a bit of time.

Eventually answering my own question:

Thanks to Berkay Gökmen for the detailed and nicely formatted answer, but when I begun going through these steps, I feared that these installations would temper with Linux Mint.

I do not recall precisely but at one of the steps, a python command meant for python3.9 would have happened with python3.8, the version of Python that is used by Linux Mint and which should not be tempered at all with.

In any case, I simply accepted the fact that the Python installed on Linux Mint is meant for the distro and not for me, the User.

If I wanted to use Python, I should use virtual environments.

Thus I chose to install pyenv and venv (virtual environment) for pyenv. I followed these instructions: https://forums.linuxmint.com/viewtopic.php?t=362499 and the instructions directly given by the packages concerned: pyenv and venv.

It is sad that Linux Mint simply takes control of Python on a machine, instead of using its own dedicated environment and leaving the global Python to the User. I think this breaks one of the mantras of Linux by putting the Operating System in the first place before the User.