[SE-4378] Fix GitlabCI builds: reformat code, fix the setup, fix any error

TODO: use template

• fix some of the GitlabCI errors related to formatting (please use a different MR for all formatting changes)
• See failed pipelines: https://gitlab.com/opencraft/billing/accounting/-/pipelines/280873379, another one, … They should work
• Coordinate with #8 (closed) if needed (e.g. if upgrading Django first would make this task much easier, then let's do the upgrade first).