Make python dependencies pypi only

This project uses pixi as dependency manager, and the dependencies are handled via conda-forge. For python development, we want to maintain an environment that only uses pypi dependencies. Investigate if it is possible for this project to migrate all python dependencies to the [project.dependencies] table in pyproject.toml, and remove them from [tool.pixi.dependencies] and [tool.pixi.pypi-dependencies]. pixi should pick up pyproject.toml for the dependency management, so that no double bookkeeping is required by the developers.

The only expected dependencies in the pixi config are python itself, and the project as an editable pypi-dependency.

Start by comparing the Python dependencies in pyproject.toml with the [tool.pixi.dependencies] and [tool.pixi.pypi-dependencies] tables. Check whether pixi resolves the project from [project.dependencies] as described, then verify that the pixi configuration retains only Python and the editable project dependency. Done means the dependency declarations have no duplicate bookkeeping and the project environment still resolves successfully.