Wanted to do this for a long time. Having two config files is just unnecessary since pyproject.toml is now the recommended config file for tools, linters, packaging, etc.

So this PR migrates all the tools to use pyproject.toml instead. There are only 3 remaining:

• license-files: Seems like this breaks test_meta.py? Otherwise it's possible to move it to pyproject.toml according to their docs.
• sphinx-pypi-upload: This was added by leandro 8 years ago, back when docs were uploaded not to RTD, but to pythonhosted.org. I recommend removing that dependency and from the setup.cfg (upload_sphinx, build_sphinx). More information on that method can be found here.
• flake8: Unfortunately, flake8 does not support pyproject.toml due to some weird reason (shame!) - pyproject.toml (PEP 518) support PyCQA/flake8#234 but we should be able to remove flake8 once ruff becomes stable.