Remove the TraitListObject, TraitDictObject and TraitSetObject cases from default value inference

[Kayvan-Zahiri]

Fixes #1591.

Following @mdickinson's review, this removes the three branches in _infer_default_value_type that mapped TraitListObject, TraitDictObject and TraitSetObject to the trait_*_object default value types. Those types only work with a List, Dict or Set handler, and List, Dict and Set set their default value type explicitly, so the branches only ever applied to other traits, where reading the attribute failed. These objects now get what plain containers get from the same inference: list_copy, dict_copy, and constant for sets. The earlier Trait()-specific change is reverted.

Behavior note: lists and dicts are now copied per instance. Sets are still shared, since there is no set-copy default value type.

Tests: the issue's add_trait example, list, dict and set defaults through Trait(), and a plain TraitType subclass (the example from the review) all error on main and pass here. Full suite: 1633 tests OK, flake8 clean.

Written with AI assistance. The runs above are real.

[mdickinson]

@Kayvan-Zahiri Thanks for the PR and the analysis.

Some project context: Trait itself is ancient and not really something we want to support; we've been edging (very) slowly towards being able to remove it altogether. Unfortunately, it's still tangled up with existing machinery (like add_trait), so we're not there yet. But the main takeaway is - we don't really want to add complication to the codebase to cater for something that's very much legacy code.

I think there's another potential way to solve this issue: instead of adding extra conditions, we remove the three branches here. Those branches are the cause of the 5.2.0 -> 6.0 regression noted in the issue; they're not really useful, and they're the cause of this issue as well as potentially other ones.

An example of a related bug that isn't touching the legacy Trait code, and that would also be fixed by deleting the three branches:

from traits.api import HasTraits, List, TraitType

class Source(HasTraits):
    values = List([1, 2])

class MyType(TraitType):
    pass

class A(HasTraits):
    value = MyType(Source().values)

A().value

The above currently raises TypeError, with roughly the same cause.

[Kayvan-Zahiri]

Thanks, that's a cleaner fix. I've pushed it: the three branches are gone, the Trait() change is reverted, and your MyType example is now a regression test next to the four from before. All five error on main and pass with the change, and the full suite passes (1633 OK). I updated the description to match.