[issue46896] add support for watching writes to selected dictionaries

[Carl Meyer]

Carl Meyer <carl at oddbird.net> added the comment:

I've updated the PR to split `PyDict_EVENT_MODIFIED` into separate `PyDict_EVENT_ADDED`, `PyDict_EVENT_MODIFIED`, and `PyDict_EVENT_DELETED` event types. This allows callbacks only interested in e.g. added keys (case #2) to more easily and cheaply skip uninteresting events.

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<https://bugs.python.org/issue46896>
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