Lower the Python3 version

Solution 1:

Don't mess with, remove or replace the preinstalled Python interpreters!

The easiest option for Ubuntu LTS releases is to add the deadsnakes PPA which provides the latest of each minor Python version (3.6.x, 3.7.x, 3.8.x, ...) as regular packages, so it integrates well with your system. Once installed, you can call those versions with the python3.6 etc. command instead of plain python3.

Alternatively, you can use e.g. pyenv to manage multiple independent interpreter versions for your development projects. Find it here, or use the simple installer. This gives you more flexibility and control, but is also a bit more effort and complex IMO.

Yet another possibility would be to use a containerization technology such as Docker to have always the same, reproducible and isolated build and run environments for your development.

Besides though, most not too special Python 3 code is compatible with newer interpreters, unless you use specifically deprecated methods and features. It's always good to keep testing your projects against newer releases and adapt/fix them as needed to keep them alive.