Empty long description will not create "UNKNOWN" as before, which breaks twine check

[hzhangxyz]

Description

When I use github action to build my package, the wheel file fail to upload to pypi.
because cibuildwheel create metadata like

Metadata-Version: 2.1
Name: PyTAT
Version: 0.2.14
Summary: python binding for TAT(TAT is A Tensor library)
Home-page: https://github.com/hzhangxyz/TAT
Author: Hao Zhang
Author-email: zh970205@mail.ustc.edu.cn
License: GPLv3
License-File: LICENSE.md
Requires-Dist: numpy

But this is what it output before

Metadata-Version: 2.1
Name: PyTAT
Version: 0.2.13
Summary: python binding for TAT(TAT is A Tensor library)
Home-page: https://github.com/hzhangxyz/TAT
Author: Hao Zhang
Author-email: zh970205@mail.ustc.edu.cn
License: GPLv3
Platform: UNKNOWN
License-File: LICENSE.md
Requires-Dist: numpy

UNKNOWN

Without the sentence "UNKNOWN", twine check raise an error saying

ERROR    `long_description` has syntax errors in markup and would not be rendered on PyPI.
         No content rendered from RST source.

Build log

https://github.com/hzhangxyz/TAT/actions/runs/2479911857

CI config

https://github.com/hzhangxyz/TAT/actions/runs/2479911857/workflow

[henryiii]

I expect this is a change in Setuptools and probably a bug in twine.

[hzhangxyz]

I expect this is a change in Setuptools and probably a bug in twine.

You are right, PEP 566 says "In addition to the Description header field, the distribution’s description may instead be provided in the message body".