Drop Python v3.5 support

Date when Python project won’t support v3.5

• Python 3.5 support has been dropped September 30th of 2020.

Other projects dropping Py 3.5

• humanize Python 3.5 drop support

Todos

DuniterPy

• duniterpy#60 (closed)

Silkaj

• Add back Black dependency to pyproject.toml and poetry.lock
• Remove black installation from Docker images v3.8
• Remove tests-3.5 job from .gitlab-ci.yml
• duniterpy#60 (closed): Update to DuniterPy v0.60.0 which drops Python 3.5 support
• #223 (closed), #354: Add ModuleNotFoundError exception.
• #194 (closed): 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.