Also, FWIW:
[build-system]
requires = []
python-path = [".", "vendored"]
build-backend = "setuptools.build_meta"
Is how I would envision it working for setuptools if setuptools choose to just bundle wheel as a build dependency rather than seeing about the possibility of merging the two projects or otherwise worked out how to build at least itself with zero dependencies.
This is pretty close to how flit already bootstraps itself so it shouldn’t be an issue there, (although it currently falls under the implicit “danger” of a self hosting build backend having any build dependencies where if say requests switched to flit).
I think those are the only two build backends in major use today.