I misread from another part of the documentation: Creating releases
The main mechanism replacing setup.py is PEP 517. I’m not familiar with these tools which compile extension modules, however enscons and the mesonpep517 you suggested look promising.
Yes, I was thinking a subprocess then a copy relative to __file__. A far cry from the declarative intentions of setup. I did mean to say Distribution, not Extension.
On that, using setup.py is most likely not the 2021 approach, and it seems the ecosystem is moving toward solving extension-module development via PEP 517. For now, as I said above, the most support is for the setup.py method (with cffi, Cython, etc)