pyproject.toml guide gives misleading advice on license field for all extant versions of setuptools

[SnoopJ]

Issue Description

Note: I am filing this issue on behalf of an IRC user who asked for help debugging the resulting mess in #python on Libera.chat.

The Writing your pyproject.toml guide was updated in a8f6ba6 to reflect the standards changes induced by PEP 639, namely the inclusion of support for SPDX expressions in the project.license field and the deprecation of table values for this field.

As far as I can tell, support for this has not yet landed in setuptools, meaning that a setuptools user reading the guide today cannot possibly get a correct pyproject.toml unless they suss out this lag between the PyPA guidance and what setuptools supports, and discover the deprecated syntax from somewhere else.

I'm not sure what the best resolution is here, the PyPA guide is justified in not waiting on setuptools and in removing the now-deprecated syntax, but it's also pretty confusing to nudge the reader in the direction of setuptools and then give them incompatible metadata syntax. Perhaps a warning infobox would be appropriate that setuptools still supports only the deprecated declaration, along with a link into the relevant portion of the setuptools user guide

Code of Conduct

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