FIX: Remove setuptools from run dependencies

[matthewfeickert]

Description

setuptools is not used at runtime and should not be required as a runtime dependency. It is a build time dependency only to provide a build-backend.

Relevant for conda-forge/openmc-feedstock#85.

https://github.com/openmc-dev/openmc/blob/develop/CONTRIBUTING.md didn't explicitly say that all PRs need to have a motivating issue first, but if that is expected I can open an Issue.

Checklist

• I have performed a self-review of my own code
• [N/A] I have run clang-format (version 15) on any C++ source files (if applicable)
• [N/A] I have followed the style guidelines for Python source files (if applicable)
• [N/A] I have made corresponding changes to the documentation (if applicable)
• [N/A] I have added tests that prove my fix is effective or that my feature works (if applicable)

[matthewfeickert]

@paulromano @shimwell @MicahGale As you were the maintainers to touch pyproject.toml in 2025 I'm tagging you for review. Let me know if you have any questions.

[shimwell]

Many thanks, and also thanks for your efforts on the conda forge package

let us leave this for a day in case anyone has comments.

We can merge tomorrow if there are no objections

[paulromano]

Thanks for catching this @matthewfeickert! We used to use pkg_resources a while back but have since switched to the standard-library importlib so removing setuptools as a runtime dependency is indeed safe.