Drop Python v3.6 support

Date when Python project won’t support v3.6

• Python 3.6 support will be dropped in December 23th of 2021.
• One important platform we are dropping is RHEL/CentOS7, which still uses Py3.6.

Todos

DuniterPy

• Bump minimum version in pyproject.toml
• Delete test job for v3.6 from .gitlab-ci.yml
• Upgrade pyupgrade to --py37-plus argument in pre-commit hook
• Update documentation

Container

• docker/python3/poetry#1 (closed): Consider deleting v3.5 image generation, if not used by any maintenance releases.
• Delete v3.6 Docker image generation. Can be removed when no Silkaj or DuniterPy maintenance releases support are stopped.