pyproject.toml missing

The minimal pyproject.toml for setuptools projects should be:

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

See https://setuptools.pypa.io/en/latest/userguide/quickstart.html#basic-use

You could also think about moving all options from setup.cfg to pyproject.toml to get rid of one file, see the template repository:

https://gitlab.cta-observatory.org/cta-computing/documentation/python-project-template/-/blob/main/pyproject.toml?ref_type=heads