Update: There is now a C version of the `blake3` Python module available at https://github.com/oconnor663/blake3-py/tree/master/c_impl. It's completely API-compatible with the Rust version, and it passes the same test suite. Multithreading (which is implemented in upstream Rust but not in upstream C) is exposed through a "max_threads" argument, as Larry Hastings suggested. The C implementation allows this argument but ignores it.

Unlike my previous attempt, this setup.py build handles the full range of target platforms and optimized flavors: x86-64 assembly on Windows-MSVC and Unix, 32-bit x86 intrinsics on Windows-MSVC and Unix, NEON intrinsics on AArch64, and portable C for everyone else. I'm new to distutils/setuptools and not particular familiar with the MSVC toolchain either, so there's a good chance that others can suggest better/cleaner/more robust approaches than what I've got, but it's at least working on my machines and on GitHub CI. (I haven't tried to get any cross-compilation working though; is that a thing?)

I haven't published this module to PyPI, partly to avoid confusion with the Rust-based implementation, which I think most applications should prefer. But if it would make a big difference to anyone who wants to review this code, we could certainly put it up as `experimental_blake3_c` or something? Let me know what's best.