gh-124872: Change PyContext_WatchCallback to take PyObject

[rhansen]

The PyContext struct is not intended to be public, and users of the API don't need anything more specific than PyObject. Also see gh-78943.

I don't think a NEWS blurb is needed because this amends a feature that is new to v3.14 so the existing blurb should suffice.

cc @fried

• Issue: Add c-api to set callback function on enter and exit of PyContext #119333

• Issue: Replace context enter and exit events with single "context switched" event #124872

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📚 Documentation preview 📚: https://cpython-previews--124737.org.readthedocs.build/

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[bedevere-app]

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[bedevere-app]

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If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[1st1]

@rhansen please rebase

[1st1]

Good refactoring and jives well with the idea that C API should expose (PyObject*) for objects and not their concrete structs.