gh-106535: Document soft deprecations in What's New In Python 3.13

[vstinner]

• Issue: Document PEP 387 Soft Deprecation #106535

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📚 Documentation preview 📚: https://cpython-previews--106859.org.readthedocs.build/

[serhiy-storchaka]

I think there is a large difference between these cases. PyModule_AddObject() is broken by implementation and cannot be fixed. It will be deprecated at compile time in future, but perhaps kept for a time. On other hand, getopt is a simple module with an interface familiar to users of other programming languages. It is a cross-language standard. I think it should never be removed.

[vstinner]

If later, PyModule_AddObject() follows a regular deprecation with planned removal, it can be documented like that. Now it's soft deprecated, so i just announce it in What's New In Python 3.13.

[vstinner]

Merged, thanks for the review Serhiy.