classifier vs classifiers in the upload api

[konstin]

Describe the bug

The upload api used by twine generally accepts fields from the core metadata specification. But while the core metadata describes a field called Classifier, warehouse ignores classifier and instead wants classifiers. It seems that PEP 301 has introduced this mismatch, and setuptools/twine have kept it since.

Expected behavior

Since I don't expect that the fields of the upload api can be changed at this point, it would be good if there was some documentation about where the upload differentiates from the core metadata specification.

To Reproduce

Try uploading a package with classifier in the POST form-data, find that it doesn't show up in the ui.

My Platform
n/a

Additional context

This was first reported in PyO3/maturin#407

[di]

Closing as a duplicate of #3151.