[flake8-pyi] Respect pep8_naming.classmethod-decorators settings when determining if a method is a classmethod in custom-type-var-return-type (PYI019)

[AlexWaygood]

Summary

I noticed that PYI019 was using its own routines for determining whether a function was a classmethod or not, when most of our linter rules use ruff_python_semantic::function_type::classify() for this purpose. Using the common method in PYI019 means that this rule will benefit from any bugfixes we make to classify() in the future, and also means that this rule now respects the pep8_naming.classmethod-decorators setting when deciding whether a method is a classmethod or not.

Test Plan

cargo test -p ruff_linter

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.