Fix #1: Moved from Pipenv to Poetry for dependency management
The `setup.py` was removed because it's generated by Poetry when necessary. The `pyproject.toml` file contains the minimal versions of the dependencies, which are set to the current release versions. This code may be compatible with older versions as well, but that hasn't been tested. IMO it's a good idea to have explicit minimal versions in this file (compared to using "*" as we did before).