Release 4.1.0

[elbersb]

Summary

• Modernize build infrastructure: migrate to pyproject.toml, remove setup.py/setup.cfg/Makefile
• Switch to ruff for linting and formatting, add ty for type checking
• Use uv and tox-uv for faster CI and local development
• Add type annotations throughout the codebase (Optional parameters, return types, abstract methods)
• Remove dead code: unused functions, constants, and commented-out class stubs
• Fix pandas deprecation warnings
• Remove BetaBinomial.as_cumulative() classmethod (unused, untested)

Test plan

• uv run tox -p auto — all 4 environments pass (py39-min, py310, py311, py312)
• uv run ruff check — passes
• uv run ruff format --check — passes
• uv run ty check — passes
• python -c "import spotify_confidence; print(spotify_confidence.__version__)" — prints 4.1.0

[jsemb]

Readme release badge also needs an update 😄

[elbersb]

done