uv<0.6 releases used requires.txt from egg-info of the sdist as a
fallback. This has been changed. Since Salt requires its legacy
setup.py, which in turn requires setuptools<69, it could not be
built properly and its dependencies were never installed.

This constrains setuptools for all package builds. In theory, we
could limit this to the installation of Salt in noxfile.py, but the
--build-constraint parameter requires a file, which would need to be
included or created ad-hoc. Constraining setuptools in this way should
not cause problems in the near future, otherwise we can go this more
complicated way. I might have missed something since I'm not a Python
packaging pro. Better suggestions are welcome.

Edit: Turns out it's not uv's fault as evidenced by pip having the same problem. It's caused by setuptools>=75.6.0. I'll quote the corresponding message on the Saltstack Discord:

Okay, I traced this back to setuptools 75.6.0 (75.5.0 still works). Comparing the changes (pypa/setuptools@v75.5.0...v75.6.0), I would guess it's caused by this commit (not an expert here): pypa/setuptools@fbbfbfb (from pypa/setuptools#4701), specifically removing the call to pkginfo_to_metadata (which reads the sdist salt.egg-info/requires.txt: https://github.com/pypa/setuptools/blob/bf2ced2a61833915a307c73405da99b6408154c7/setuptools/_vendor/wheel/metadata.py#L155-L163) in favor of just copying the sdist PKG-INFO to the wheel METADATA.