add python 3.12 support

[thrau]

Various changes to make plux 3.12 compatible.

• add setuptools to dev dependencies (needed by the CLI)
• vendor two pkg_resources utils functions and remove it as dependency
• replace distutils imports with appropriate setuptools imports
• use metadata.Distribution to correctly resolve entry_points.txt

This PR also shows that a lot of modernization and cleanup is needed to better separate build tools (setuptools) from runtime tools (metadata&importlib), and use the current APIs correctly.