Merge branch 'feature-fix-ci' into 'master'

 Fix flake8 dependencies and  Upgrade CI

Automatically remove flakehell dependencies from `mkdocs_template_rdeville` as
flakehell seems to be dying.
Replace flakehell by flake9, fork of flake8 with pyproject support.
Update files accordingly.

Fix pydocstyle warning in plugins.py files.

Upgrade CI from mkdocs_template_rdeville.

See merge request rdeville.private/my_programs/mkdocs_template!3