Generate pyproject.toml

[Mandera]

ManderaGeneral/generalimport#18 (comment)

Maybe leave setup.py with a shim? https://stackoverflow.com/questions/62983756/what-is-pyproject-toml-file-for

[Mandera]

Maybe I could deprecate metadata.json and only use this?

[Mandera]

Had issues with pip install -e .
Found here that you can do pip install -e . --config-settings editable_mode=compat.
That works but writes a warning

DEPRECATION: Config settings are ignored for project file:///C:/Code/Repo/generallibrary. pip 23.3 will enforce this behaviour change. A possible replacement is to use --use-pep517 or add a pyproject.toml file to the project.