Move away from pipenv

[femueller]

When the project was created pipenv just emerged and seemed quite popular. From my observation this doesn't seem to be case anymore. Therefore, I would suggest to move this project also either to the traditional setup way (requirements.txt + requirements-dev.txt) or Poetry.

The benefit for having again a more mainstream approach would making it easier to contribute to the project.

[markusressel]

My suggestion would be to use poetry, since I believe it can be used for both library as well as application projects and since this repo is kinda both, I think it would fit well. I believe it can also be used to generate a requirements.txt, although I don't think it's necessary when using poetry.