gh-124153: Add Py_tp_token support to xxlimited module

[gpshead]

Summary

Implement the TODO at line 99-100 in Modules/xxlimited.c to add Py_tp_token slot support, demonstrating Python 3.14 type checking best practices for Limited C API extension modules.

Changes

• Update Limited API version from 3.13 to 3.14
• Add Py_tp_token slot with Py_TP_USE_SPEC to Xxo_Type_slots
• Add "Type Checking with Heap Types" section to isolating-extensions.rst

Rationale

This addresses the type-checking problem for heap types described in PEP 630. The xxlimited module serves as a template for extension developers, and this change demonstrates modern Python 3.14 type-checking best practices using the Py_tp_token feature added in #124153.

Relates to #134160 (documentation modernization).

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PR description written with Claude Code.

• Issue: Add PyType_GetBaseByToken function with Py_tp_token slot #124153

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📚 Documentation preview 📚: https://cpython-previews--141644.org.readthedocs.build/

[gpshead]

I guess my question about this is... should we recommend people always do this? is there a downside worth mentioning? cc @neonene as well as the original author of the 3.14 feature PR.

[encukou]

Thanks for adding this!

should we recommend people always do this

IMO, no -- they should only do it if they need the token (e.g. for XxoObject_Check). Ideally, this should be a tool that's there if you need it, not “best practice” to be cargo-culted in every definition.

But, on the other hand, it's not easy to document it that way. So, maybe suggesting that people should do it, and putting it in the copy-pastable example is the right way.
But then people might wonder why Python doesn't do this automatically...

[encukou]

What do you think about copying the original to xxlimited_3_13.c, so we have an example for older stable ABI?

[encukou]

#142827 adds a Xxo_state_from_type function and several examples of getting state->Xxo_Type from that. It also removes the TODO.

After/if that's merged, I plan to adapt the HOWTO addition from this PR after that. And ideally add an actual example of type-checking :)