Preparing migration to Poetry

[femueller]

Part of: https://github.com/femueller/python-n26/issues/121

Done using Dephell:

dephell deps convert --from=Pipfile --to-format=poetry --to-path=pyproject.toml

What's missing:

• [x] PyPI releases via Poetry
• [x] Modify Dockerfile to use Poetry
• [x] Modify Makefile to use Poetry

[femueller]

Once the PyPI release process works via Poetry, we also don't need to generate a requirements.txt anymore.

[femueller]

I went with this GitHub Action as it seems to be actively maintained and the most used for poetry publish. Still need to verify if it works as expected.