feat: make Python 3.7 the minimum version
- Python 3 version of Debian 10 (current stable) is 3.7. https://packages.debian.org/buster/python3
- Python 3 version of Ubuntu 20.04 (current LTS) is 3.8. https://packages.ubuntu.com/focal/python3
De facto, we don't test anymore on Python 3.6 and we won't change this. In consequence, let's officially drop support for 3.6, that might already be broken and that we would probably not want to fix if it was the case.