The deadsnakes PPA make the latest stable versions of python available on Ubuntu 18.04 LTS (3.9 is already in the official apt repo for 20.04). I now find it preferable to installing from source.
The following was tested on a Ubuntu 18.04.5 LTS desktop with python 3.6.9 as the system python version ("python3").
The latest python available from deadsnakes as of July, 2021 is 3.9.6.
