Building Python projects with Buck2

Speedrun:

1. Install buck2 (cargo +nightly-2024-02-01 install --git https://github.com/facebook/buck2.git buck2 or fetch the latest binary release)
2. git clone https://github.com/zsol/buck-py-demo
3. Run ./cpython/gen_headers.sh to teach Buck about the CPython .h files for your local version of CPython
4. cd buck-py-demo && buck2 run :demo