build: convert setup.py based setup to pyproject.toml

[afuetterer]

This PR proposes the following changes:

• remove setup.py based project setup
• use modern python project setup with pyproject.toml
• add optional dependency group test, can be installed with pip install .[test]
• add a second CI job, that uses build-and-inspect-python-package to check the built package and see it contents at a glance, see: summary

Please let me know what you think.

Refs:

• https://packaging.python.org/en/latest/tutorials/packaging-projects/
• https://packaging.python.org/en/latest/guides/modernize-setup-py-project/
• https://github.com/hynek/build-and-inspect-python-package

[afuetterer]

Also the setup.py stated python_requires='>=3.6.0', but CI tested only from 3.7 on.

Python versions 3.6 and 3.7 reached end of life in 2021 and 2023. I would recommend to drop support and require Python >= 3.8. What do you think?

Ref: https://devguide.python.org/versions/

[afuetterer]

Hi @gtonkinhill,
is this not desired? If you don't like the change, please let me know. I will close the PR then. Thanks.