[FR] warn on packaging a `py.typed` marker but missing the `Typing :: *` classifier

[Avasam]

What's the problem this feature will solve?

A typed project or partial stubs-only package will ship a py.typed marker as per https://typing.python.org/en/latest/spec/distributing.html#packaging-type-information

But it's easy to forget to also add the appropriate Trove Classifier for discoverability. The setuptools build backend already outputs warnings in some scenarios (for example: deprecated classifiers). It feels well within scope to warn about incorrect or missing classifiers.

Describe the solution you'd like

warn on packaging a py.typed marker but missing the Typing :: * classifier

In this case, having either Typing :: Stubs Only or Typing :: Typed should be enough to avoid the warning I'm suggesting.

Inversely, having the classifier in a package that doesn't ship a py.typed file OR ends in -stubs (the package module name, not the distribution) should also be a warning.

Alternative Solutions

This is different than #1021 as this isn't auto-inserting anything, just showing a non-blocker warning.

Additional context

No response

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