> In setup.py I assume that the target platform of the build is the same as the current interpreter's platform.

If this is included in CPython, it won't be using setup.py, so this isn't a concern.

I don't think there's a way to use setup.py to cross-compile, so I'm not sure this ever was a concern.


> - Compiling assembly files.

AFAICT Python currently ships exactly one assembly file, "Modules/_decimal/libmpdec/vcdiv64.asm", which is only built on Windows.  It would be a brave new world of configure.ac hacking to build assembly language files on POSIX platforms.  As a first pass I say we merge the reference C implementation.  Maybe someday we could add the SIMD assembly language stuff--or use the one built in to OpenSSL (if they ever add BLAKE3).

> I assume we don't want to check in the .obj files?

Correct, we don't.

> - blake3module.c contains an awful lot of gotos to handle allocation failure cases.

Works for me, please keep it.