chore: use scikit-build-core for the build

[henryiii]

Description

Switches to scikit-build-core from setuptools, similar to nanobind. Using PEP 639 licence. The version is read from the C++ files. The CMake files are generated in the wheel step for now. Setup to make the global SDist is a bit different.

Suggested changelog entry:

* Use scikit-build-core for the build backend. The CMake generation has been moved to the sdist->wheel step. ``PYBIND11_GLOBAL_PREFIX`` has been removed.

[jcfr]

re: Suggested changelog entry

Explain how results previously obtained by setting PYBIND11_GLOBAL_PREFIX can now be achieved. (e.g by running nox -s build_global or by installing a pybind11-global SDist)

[henryiii]

As for the explanation, I think that should go in the upgrade guide.

[Copilot]

Pull Request Overview

This PR transitions the project’s build system from setuptools to scikit‐build‐core, removing deprecated setup scripts and updating tests to accommodate the new backend. Key changes include:

• Removing the build-system block from tools/pyproject.toml and updating the main pyproject.toml with scikit‐build‐core settings.
• Removing setup.py and associated legacy build logic.
• Adding tools/make_global.py and adjusting tests and nox sessions to support the new global build process.

Reviewed Changes

Copilot reviewed 11 out of 17 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tools/pyproject.toml	Removed legacy build-system configuration for setuptools.
tools/make_global.py	New script for generating a global configuration for sdist/wheel.
tests/extra_python_package/test_files.py	Updated tests to work with the new build backend and file layout.
setup.py	Removed in favor of scikit‐build‐core.
pyproject.toml	Rewritten to use scikit‐build‐core with updated project metadata.
pybind11/_version.py	Adjusted version extraction using regex from the header file.
noxfile.py	Updated nox sessions to support both standard and global builds.
docs/conf.py	Updated to use pathlib for loading version information.
.pre-commit-config.yaml and workflows files	Minor changes to remove outdated hooks and extra args.

Files not reviewed (6)

• CMakeLists.txt: Language not supported
• MANIFEST.in: Language not supported
• setup.cfg: Language not supported
• tests/requirements.txt: Language not supported
• tools/setup_global.py.in: Language not supported
• tools/setup_main.py.in: Language not supported

Comments suppressed due to low confidence (1)

tests/extra_python_package/test_files.py:237

• [nitpick] Verify that the differences in pkg_info assertions between global and non-global builds are intentional and well-documented, ensuring consistency in the expected package metadata.

assert "Provides-Extra: global" not in pkg_info

[cryos]

This looks good to me. Nothing jumping out as an issue, CI looks good.