Modernize project packaging: migrate to pyproject.toml

[dguido]

Summary

This PR modernizes the project's packaging infrastructure by migrating from the legacy setup.py to the modern pyproject.toml standard, using hatchling as the build backend.

Motivation

Issue #52 requested switching to pyproject.toml as it has become the standard for Python packaging. This brings the project up to date with modern Python packaging best practices.

Changes

• Replaced setup.py with pyproject.toml: Using hatchling as the build backend (the modern successor to flit)
• Added ruff configuration: Comprehensive linting and formatting rules configured directly in pyproject.toml
• Simplified version.py: Made it compatible with modern tooling while maintaining backwards compatibility
• Updated .gitignore: Added common Python development patterns and tool caches
• Removed MANIFEST.in: No longer needed as hatchling handles file inclusion through configuration

Key improvements

• ✅ Modern packaging standard (PEP 517/518/621 compliant)
• ✅ Better tooling support (works great with uv, pip, pipx, etc.)
• ✅ Integrated linting configuration with ruff
• ✅ Simplified version management
• ✅ Added Python 3.11 and 3.12 to classifiers
• ✅ All dependencies and metadata preserved exactly

Testing

• The pyproject.toml has been validated and parses correctly
• All project metadata, dependencies, and entry points are preserved
• Version information remains accessible through the simplified version.py

Backwards Compatibility

This change maintains full backwards compatibility:

• All dependencies remain the same
• The package name, version, and metadata are unchanged
• Console script entry point remains graphtage
• The package can still be installed with pip install . or pip install graphtage

Closes #52