python3-setuptools-rust-opt (setuptools-rust installed to /opt)

setuptools-rust is a plugin for setuptools to build Rust Python
extensions implemented with PyO3 or rust-cpython.

Installs to /opt/python$PYVER/site-packages

Add the following before the python3 build commands in your
Slackbuild to use this version.:

PYVER=$(python3 -c 'import sys; print("%d.%d" % sys.version_info[:2])')
export PYTHONPATH=/opt/python$PYVER/site-packages