He wouldn't need to use MANIFEST.in, just fixing setup.cfg would be enough. The simplest fix would be to set py_modules = communism and remove packages from setup.cfg altogether (and then the __init__.py file in src/ could be deleted since src/ is not a package).
Author of the project here. I took very little time to write this joke, this obviously an oversight from me. I was working on my conda installation, so I didn't look further.
Yes, it works now. Note that since you have no packages, you can omit include_package_data = True from setup.cfg, and since you no longer have the src/ directory, you can omit the options.packages.find section from setup.cfg.