gh-141226: Deprecate PEP-456 support for embedders

[picnixz]

• Issue: Deprecate Py_HASH_EXTERNAL as it cannot be used for --with-hash-algorithm #141226

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📚 Documentation preview 📚: https://cpython-previews--141287.org.readthedocs.build/

[hugovk]

Good spot, we'll also need to backport this fix.

[picnixz]

Do you prefer that I open a separate PR for that? I can definitely do so for miss-ilington.

[hugovk]

Any reason to rush this and not use the preferred five years?

[picnixz]

Err.. I forgot the 5 years. I did a +2 version but I can definitely do a +5 years. However, note that I couldn't find any (public) code that was using this feature so I guessed we could drop this.

Actually, as I told Victor, the maintenance cost on our side won't change at all because we already have all the infrastructure in place except for actually enabling this feature during the build (but all the code is already there and is straightforward). What is however hard to maintain are the docs for that (it's not easy to explain what needs to be done) and if users use this feature wrongly, it becomes a really bad footgun.

I think "advanced users" (embedders) would rather patch the entire file instead of relying on this as our configure script doesn't even support this...

[picnixz]

I decided to push the deprecation to 3.19. That way, if people are actually using that feature by directly passing the CFLAG and manually link their own definition, we have time to add the correct configure support instead.

[vstinner]

I'm +1 on deprecating this feature. I have no opinion on which release should remove the code.