Fix .pth file installation location in wheels

Resolves issue where pip_system_certs.pth was installing to venv root
instead of site-packages directory, preventing the package from working
when installed from wheel.

Changes:
- Add minimal setup.py with BuildWithPth command to place .pth file correctly
- Update pyproject.toml to use legacy backend for custom command support
- Remove problematic package-data configuration from pyproject.toml
- Update test script to use modern build tools (python -m build)
- Remove obsolete setup.cfg universal wheel configuration

The .pth file now correctly installs to site-packages/pip_system_certs.pth
where Python can process it during startup for truststore injection.

🤖 Generated with [Claude Code](https://claude.ai/code

)

Co-Authored-By: Claude <noreply@anthropic.com>