Follow-up from #31132:

It has been reported that it's still leaking with the upgraded python3 from homebrew that includes Homebrew/homebrew-core@784d292

The sagelib build collects many -I, -L directives through pkgconfig.
Such directives on the command line take precedence over the search paths set by sage-env in $CPATH, $LIBRARY_PATH.
Now if -I/usr/local/ or -L/usr/local (instead of more specific paths to /usr/local/Cellar/.../...) appear, then unwanted packages that happen to be installed can leak in.

However, here the problem is caused by homebrew installing a distutils.cfg that injects /usr/local/include into our include search path.
We fix the leak by switching to a modern configuration of setuptools.

This also affects cysignals, as reported in https://groups.google.com/g/sage-release/c/KdSKg6RdZok/m/91Ibwn8RAgAJ

Relevant tickets: #13348, #14709, #29562 (+), #29607 (+), #29697 (?), #31041 (), #30818 (), #30013 (~); possible follow-up: #31338

CC: @jhpalmieri @zlscherr @kiwifb @kliem @slel

Component: build

Author: Matthias Koeppe

Branch: e03c5ac

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/31335