Followup to #2666 (comment) . As @di said,

Metadata is specified at https://packaging.python.org/specifications/core-metadata/ but the conversion between this and "what fields PyPI accepts" is not documented. Perhaps we need more information here: https://warehouse.readthedocs.io/api-reference/legacy/#upload-api