[flake8-bugbear] Ignore enum classes in cached-instance-method (B019)

[charliermarsh]

Summary

While I was here, I also updated the rule to use function_type::classify rather than hard-coding staticmethod and friends.

Per Carl:

Enum instances are already referred to by the class, forming a cycle that won't get collected until the class itself does. At which point the lru_cache itself would be collected, too.

Closes #9912.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.