gh-111178: fix UBSan for custom.c examples

[picnixz]

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178

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📚 Documentation preview 📚: https://cpython-previews--131606.org.readthedocs.build/

[vstinner]

LGTM

cc @encukou

[encukou]

These are included in the docs (newtypes_tutorial.rst), which redundantly define most of those in the prose, and have some notes that reference the casts. Could you change those as well?
(Perhaps there's a way to consolidate this with something like Sphinx's :start-at:, but that might be a distraction right now. In any case, some of the actual prose does need to be updated.)

[picnixz]

I observed that I incorrectly called a function as well, so thanks Petr for pointing out the docs.

[picnixz]

@encukou feel free to merge this one tomorrow or whenever you want, I'm not on my dev session so I can only edit from the web UI.

[encukou]

Thanks!
FWIW, there's also newtypes.rst with some UB examples.