Move modules from namespace 'mlx' to 'mlx/xunit2rst'

[JokeWaumans]

1. The modules no longer clutter the namespace mlx. They have been moved to the subpackage mlx.xunit2rst.
2. Convert the pkg_resources-style namespace package to a native namespace package
3. Remove support for Python 3.7
4. Test support for Python 3.12
5. Address deprecation warning about use of pkg_resources
6. Modernize the build system:
   • Replace python setup.py with python -m build
   • Add a basic pyproject.toml