Summary

• add a packaged pypfopt.data module with loaders for bundled example prices, SPY prices, and Black-Litterman market caps
• update the 5 cookbook notebooks to load bundled example data instead of downloading with yfinance
• remove yfinance from the dev extra and add tests for the new data loaders

Why

Issue #716 tracks the cookbook notebooks' dependency on live yfinance downloads. That makes notebook CI depend on external network calls and changing upstream data. Bundling the example datasets keeps the notebooks reproducible and allows nbmake to run offline.

Impact

• cookbook notebooks now run against stable packaged example data
• package distributions include the example CSV and JSON assets used by the notebooks
• developers no longer need yfinance installed to run the notebook test suite

Validation

• uv run --python .venv\Scripts\python.exe pytest ./tests
• uv run --python .venv\Scripts\python.exe pytest --reruns 3 --nbmake --nbmake-timeout=3600 -vv cookbook
• uv run --python .venv\Scripts\python.exe python -m build --sdist --wheel --outdir dist-check