I just merged https://github.com/pypa/packaging.python.org/pull/1031, which means that Hatchling is now the default build backend, and setup.{py,cfg} are nowhere to be found.
https://packaging.python.org/en/latest/tutorials/packaging-projects/
PRs with further refinements are welcome. Thanks again @henryiii and @ofek for pushing this forward, and everyone who contributed to the conversation.