setup.py still mentioned even after #169 Replace setup.py with pyproject.toml

Feature https://github.com/xxh/xxh/issues/156: Replace setup.py with pyproject.toml #169

This line still mentions the non-existant `setup.py`:

https://github.com/xxh/xxh/blob/db4a6356f98d0fe47c6fd6a0d32decd4673912fd/xde/xde#L162

What are the building instructions these days?  The Arch Linux package is broken because of referencing `setup.py`.