gh-127124: Pass optional state to context watcher callback

[rhansen]

This enables users to associate state with the callback without relying on globals.

Also:

• Refactor the tests for improved readability and extensibility, and to cover the new state object.

• Drop the pointer from the PyContext_WatchCallback typedef. This de-obfuscates the fact that pointers are involved, and makes it possible to forward-declare functions to improve readability:

  staticPyContext_WatchCallbackmy_callback; intmy_callback(PyObject*cbarg, PyContextEventevent, PyObject*obj){... }

This will conflict with #124741; if one is merged I'll rebase the other.

• Issue: Pass opaque state to PyContext_AddWatcher callback #127124

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📚 Documentation preview 📚: https://cpython-previews--127140.org.readthedocs.build/

[ZeroIntensity]

Needs a NEWS entry (even though it's a change to a new feature, we want to show that we changed it in the a3 changelog)

[rhansen]

I'll open a separate PR for the callable idea I mentioned in #127124 (comment) so that we can compare the two and decide which we like better.

Edit: Done; see #127247.

[rhansen]

Does this need a Py_VISIT somewhere? I didn't see any for the other fields in this struct, but maybe the GC does something special with this struct?

[ZeroIntensity]

FWIW, no need to force push changes--it makes reviewing harder. It's fine to just commit normally, and then everything gets squashed.

[rhansen]

Force pushing is the only way to update the commit message.

[ZeroIntensity]

Yes, but it makes reviewing more difficult. FWIW, nobody really cares about the commit messages in the PR itself, the end message that gets squashed is really what matters. (There's a section in the devguide about this, IIRC.)