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.

Todos

DuniterPy

• duniterpy#179 (closed).

Silkaj

• 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

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.

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• Create ticket for dropping Python 3.7 support