Reduce the number of materialized instance __dict__s

[markshannon]

Accessing the __dict__ attribute of an object causes the dictionary to be materialized.
This has a negative impact on performance.
We should provide means to access the __dict__ without forcing materialization, which can be used in copy, pickle and any third party code that chooses to.

I propose two new methods on object:

• __dict_copy__(self), which will be equivalent to self.__dict__.copy(), but without materializing the dictionary of objects with virtual dictionaries.
• __dict_update__(self, arg), which will be equivalent to self.__dict__.update(arg), but without materializing the dictionary of objects with virtual dictionaries.

Linked PRs

• GH-106485: "Un-materialize" __dict__s in LOAD_ATTR_WITH_HINT #106496
• GH-106485: Dematerialize instance dictionaries when possible #106539
• GH-106485: Handle dict subclasses correctly when dematerializing __dict__ #107837
• GH-106485: Create object's dict-values instead of creating __dict__, when we can. #107843

[brandtbucher]

I opened a draft PR for an alternate approach, which aggressively "un-materializes" __dict__s in LOAD_ATTR_WITH_HINT.

[kumaraditya303]

This change seems to be causing crashes on buildbots. See https://buildbot.python.org/all/#/builders/123/builds/813 as example.

[markshannon]

Two more things to fix:

• Instances created by copy.deepcopy do not get their __dict__s dematerialized.
• We shouldn't be creating dictionaries in _PyObjectDict_SetItem for objects with managed __dict__s if the key is in the shared keys.

[hugovk]

Is there more to do here, or can we close this issue?