Update `CallbackResult` type to match local vs. non-local Python events

[tzanko-matev]

According to Python's documentation some event types can be disabled and some cannot. But our current Tracer trait has the same return type for all events: CallbackResult. This means that according to our API all events can be disabled. That will lead to runtime errors for an implementation which doesn't conform to the Python docs.

https://docs.python.org/3/library/sys.monitoring.html