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](https://devguide.python.org/#status-of-python-branches).
## Todos
### DuniterPy
- [x] duniterpy#179.
### Silkaj
- [x] Bump minimum version in `pyproject.toml`
- [x] Delete test job for v3.6 from `.gitlab-ci.yml`
- [x] Upgrade `pyupgrade` to `--py37-plus` argument in `pre-commit` hook
### Container
- [x] docker/python3/poetry#1: 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.
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- [ ] Create ticket for dropping Python 3.7 support
issue