Replace setup.py with modern pyproject.toml

Modernizes the build system by replacing the complex setup.py (70+ lines with
custom command classes) with a clean pyproject.toml configuration. This
eliminates custom installation logic while maintaining all functionality.

Key improvements:
- Remove custom InstallCheck/DevelopCheck/BuildIncludePth command classes
- Use standard setuptools data-files for .pth installation
- Modern SPDX license format (BSD-3-Clause)
- Proper Python version classifiers (3.10-3.13)
- Maintains git-versioner integration
- Verified wheel builds correctly with .pth file inclusion

The new configuration is more maintainable and compatible with modern
Python packaging tools while preserving the truststore integration.

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

)

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