After having read your email, I read up and realized that I can completely remove the setup.py file and rather than performing a development installation with pip, I use poetry.
I can define the package in pyproject.toml using the path directive: https://python-poetry.org/docs/dependency-specification/#path-dependencies and then execute "poetry install".
Thank you Thomas! 😃
A side note; I am using nox and want to avoid running "poetry install" in every session, as this is very time consuming. Instead, I try to filter out the packages which needs to be installed for each session. Then I constrain this version to the poetry-managed version, as shown in the "Hypermodern Python" setup. However, this entails using pip with the --constrain option. And this sets me back once more with having pip in the equation. So, I am thinking of possible solutions for this problem and raised an issue here: https://github.com/cjolowicz/hypermodern-python/issues/115
If you have any input, please do let me know :)
Kind regards, Fredrik