I believe the SBOMS for Python packages proposal by @sethmlarson is an existing, standards compliant (supports spdx), and more developed proposal to address software supply chain data in python packages:
- SBOMs for Python packages project
- GitHub - psf/sboms-for-python-packages: Software Bill-of-Materials documents for Python packages
- Visualizing the Python package SBOM data flow
In addition the proposal posted here is limited in that it is framed around setuptools not as a general mechanism for all build backends.