setup.py as implemented with distutils/setuptools has a bit of a
Goldilocks problem: it's just right for a medium-complexity project
but when your project is very simple it's too hard, and when you get
to the point where you are trying to extend distutils by writing a
10,000 line extension, yikes. So it's fantastic to be able to just
avoid distutils entirely if it isn't the right size for your project.
This example, flit, does not invoke any code from distutils,
setuptools or bdist_wheel to do its thing.

A source release could just be an archive of the repository.