dict/func/code watchers may call dealloc callback with exception set

[carljm]

The docs at https://python.readthedocs.io/en/stable/extending/newtypes.html#finalization-and-de-allocation note that dealloc funcs need to be careful to work correctly in the presence of a pending exception, because deallocation may happen in unwinding while an exception is pending.

func_dealloc, code_dealloc, and dict_dealloc all may call watcher callbacks with a deallocation event. We shouldn't call a user callback with a pending exception set; this could cause all kinds of hard-to-debug issues. We should save/restore the exception around the callback.

[carljm]

On second thought, I think the performance cost of checking for a pending exception and then saving/restoring is too high, given that watchers are a low-level API intended for high performance. We should rather fix this by documenting that watchers may be called (in the case of dealloc events) with a pending exception set.

[carljm]

Fixed in #102382