Current Behavior

nox' test test_run_env_systemroot fails on python 3.12.8

Expected Behavior

test_run_env_systemroot should not fail on >=python 3.12.8

Steps To Reproduce

I am reporting this as a failure of a current rebuilding on NixOS that bumped python 3.12.7 -> 3.12.8.

The build logs from CI can be found here: https://hydra.nixos.org/build/281820678/nixlog/1

I tried to dig into the source code changes of 3.12.8 for shutil.which and i think the test is failing because of the following change:
python/cpython@8899e85

The change intention on cpython is to fix behavior for which on windows systems.
The test_run_env_systemroot is running shutil.which with mocked sys.platform = "win32" so should be affected by the change.

In Lib/shutil.py the line 1558 change removes cmd from the windows lookup path thus the test is failing in our ci suite.

Environment

- OS: NixOS 25.05 (nixos-unstable)
- Python: 3.12.8
- Nox: 2024.10.09

Anything else?

In addition os.pathsep (line 1553) does return an unexpected pathseperator when having the mock sys.platform = "win32" active. So i kind of have the feeling that the overall test setup is not correct when building in linux systems

I am actually not sure why sys.platform is mocked on the test at all, shouldnt it work the same on any platform?