Refactor: Get rid of setup.py

[soldni]

I propose transitioning to using just a pyproject.toml file, following PEP 621 and PEP 631.

For an example of a package that uses pyproject.toml exclusively, see pdf2sents, a component for the SCIM feature. As you can see from its pyproject.toml, all dependencies, package metadata, and even configuration for plug-ins such as black and mypy are all part of the TOML file.

Installation procedure via pip is unchanged.