Speedrun:

1. Install buck2 (cargo +nightly-2024-02-01 install --git https://github.com/facebook/buck2.git buck2)
2. git clone --recurse-submodules https://github.com/zsol/buck-py-demo (or if you've already cloned: git submodule init && git submodule update)
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
   1. If you run a Meta-internal build of buck2, you have to set the BUCK_VERSION env var to a recent version (run buck2 --version in fbsource to get one)
   2. If you run a Meta-internal build of buck2, pass --local-only to all buck2 invocations.