Python Packaging taste

# Python Packaging taste - Use `pyproject.toml` as the single source of truth and drop `setup.py` for new packages. Confidence: 0.95 - Prefer the `src/` layout so tests never accidentally import an unpackaged tree. Confidence: 0.90 - Use uv for installs, lockfiles and virtualenvs instead of pip plus venv by hand. Confidence: 0.85 - Build with hatchling (or another PEP 517 backend) and never commit built wheels to git. Confidence: 0.90 - Pin the minimum Python version explicitly and test the olde...