gh-132604: Deprecate inherited runtime checkability of protocols

[johnslavik]

• Issue: Non-runtime_checkable subclass of a runtime_checkable Protocol allows isinstance() #132604

I've chosen to go ahead with a 5-year deprecation period as Alex told me he'd do at least (thx!).
If this deprecation is too problematic, we can raise the warning eternally / soft deprecate -- and never eventually raise an exception.

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📚 Documentation preview 📚: https://cpython-previews--143806.org.readthedocs.build/

[JelleZijlstra]

Thanks!

[johnslavik]

I think we can use :deco: for runtime_checkable so I'll do that.
EDIT: Sphinx is a little limited with the markup in this regard; I've applied :deco: in the news entry and that's it.