chore: move to pyproject and poetry

[Panaetius]

closes #2405

Moves all configuration to pyproject.toml
Moves building/dependency management to poetry
Gets rid of setup.py
Updates Github Actions to work with poetry

Also:
Adds caching of python dependencies between runs in the github actions, to speed things up a bit (and reduce bandwidth)
Reduces exponential backoff in our requests wrapper so the worst case is not 17 minutes but 6seconds.
Formatter the github actions to have consistent indent
Added some missing init.py (poetry threw warnings when building)

Note:
version.py.tmpl is gone, poetry-dynamic-versioning patches version.py directly. 0.0.0 is just a placeholder. The way it works is that before a poetry build or poetry install, it patches version.py with the correct version, and after in un-patches it again. This has the downside that the version locally is always 0.0.0, only renku installed in the env and published packages have the version actually set. I didn't notice any issues with running tests or commands with this, so I think it's fine.

[Panaetius]

For an example of the cache:
Without cache: https://github.com/SwissDataScienceCenter/renku-python/runs/4264883858?check_suite_focus=true
With cache: https://github.com/SwissDataScienceCenter/renku-python/runs/4265611395?check_suite_focus=true#step:5:10 (rerun of the previous execution)

[vigsterkr]

[vigsterkr]

🚢 🚀