blake3 binary wheels / building on PyPy

[oberstet]

Bug Description

PyPI mode limitation for PyPy: The xbr package has a transitive dependency chain (xbr → py-multihash → blake3) where blake3 is a Rust extension that:

1. Has no pre-built PyPy wheels on PyPI
2. Its Rust build system reports "Unsupported platform: pp73" when attempting to build from source

Checklist

• I have searched existing issues to avoid duplicates
• I have provided a minimal reproducible example
• I have included version information
• I have included error messages/logs