Earlier quoted context omitted.
All of those setup options are now setup.cfg options: https://docs.python.org/3/distutils/configfile.html At least, that's supposed to be the case; I am not 100% certain that it always is. However, even so, AFAIK setup.cfg does not directly support using third-party libraries to compute the setup options, as your setup.py does. The only workaround I'm aware of for that if you want to get rid of setup.py is to have a…
I get the idea -- but, how do I tell Cython "it's time to compile things now" from a file I can't execute?
It looks like there are ways to specify which build tools your package requires; I would assume Cython would be one of the tools that can be specified since it's been around a while and gets a lot of usage, but I don't know for sure.