[build] Migrate build backend to scikit-build-core

[jase231]

This Pull request:

Migrates the Python build backend from setuptools to scikit-build-core, which is built for CMake. This change entirely eliminates the need for setup.py, moving all wheel build config into pyproject.toml.

Changes or fixes:

• Replaced setuptools with scikit-build-core
  • Wheel-specific config and defines are now passed directly to the build system in pyproject.toml thanks to scikit-build-core's CMake integration
• Moves alpha version warning into a version-conditional check in __init__.py to avoid injecting the warning code at build time
• Increases the ROOT wheel distribution's version to 0.1a14
• Fixes several issues that made linking against the wheel-provided ROOT installation challenging:
  • Eliminates the mock_site_packages paths that previously ended up hardcoded into CMake configs
  • Fixes mysterious missing PCM errors

Checklist:

• tested changes locally (successfully produced wheels and ran AGC without issue)
• updated the docs (not necessary as far as I can tell)

[github-actions]

Test Results

    19 files      19 suites   2d 20h 45m 42s ⏱️
 3 865 tests  3 864 ✅ 0 💤 1 ❌
66 302 runs  66 301 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 966e205.