Drop Python v3.5 support
### Date when Python project won’t support v3.5
<!--
- Still maintained: https://www.python.org/dev/peps/pep-0478/
> we expect to release Python 3.5.8 around September 2019.
- https://devguide.python.org/devcycle/#end-of-life-branches
-->
- [Python 3.5 support has been dropped September 30th of 2020](https://devguide.python.org/#status-of-python-branches).
### Other projects dropping Py 3.5
- [`humanize` Python 3.5 drop support](https://github.com/jmoiron/humanize/issues/130)
## Todos
### DuniterPy
- [x] duniterpy#60
### Silkaj
- [x] ~~Add back Black dependency to `pyproject.toml` and `poetry.lock`~~
- [x] ~~Remove `black` installation from Docker images v3.8~~
- [x] Remove `tests-3.5` job from `.gitlab-ci.yml`
- [x] duniterpy#60: Update to DuniterPy v0.60.0 which drops Python 3.5 support
- [ ] #223, #354: Add `ModuleNotFoundError` exception.
- [ ] #194: Use f-string
- [ ] …
### Container
- [ ] Delete v3.5 Docker image generation? No, keep it to support, v0.8.x which still supports Python v3.5. Can be dropped when Silkaj v0.8.x support is dropped.
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