gh-141510: Don't accept frozendict in PyDict_Watch()

[vstinner]

Don't accept frozendict in PyDict_Watch() and PyDict_Unwatch(). A frozendict cannot be modified, so it's not useful to watch for modifications.

• Issue: PEP 814: Add built-in frozendict type #141510

[vstinner]

I don't recall why I modified PyDict_Watch() and PyDict_Unwatch() to accept frozendict in my commit 696cdfc. Maybe I just replaced mechanically PyDict_Check() with PyAnyDict_Check().

A frozendict cannot be modified, so it's not useful to watch for modifications.

If later, it becomes useful, we can modify the functions again. But I prefer to start by not accepting them.

[vstinner]

cc @corona10 @Fidget-Spinner

[vstinner]

Merged. Thanks for the review @Fidget-Spinner.