Following #149 and #245, switch back to Poetry stable: - [x] in [Docker images](https://git.duniter.org/docker/python3/poetry/blob/master/3.5/Dockerfile#L9): https://git.duniter.org/docker/python3/poetry/commit/ac8190c6bd91243fa2c2336cdf28a7d1761c63bb - [x] in [Poetry installation documentation](https://git.duniter.org/clients/python/silkaj/blob/dev/doc/install_poetry.md#install-poetry)