[refurb] Allow subclassing builtins in stub files (FURB189)

[ntBre]

Summary

While looking at rule stabilization candidates for 0.16, I noticed that #15291 has been blocking
stabilization of this rule for quite a while despite being a one-line change. I decided to make that
one-line change since it seemed sensible to me. I'll still plan to hold off on stabilization until
this change has been in preview for a release cycle, but this should pave the way to stabilization
in 0.17.

Closed #15291

Test Plan

New mdtest showing that the rule is inactive in stub files

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+0 -2 violations, +0 -0 fixes in 2 projects; 54 projects unchanged)

bokeh/bokeh (+0 -1 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --no-fix --output-format concise --preview --select ALL

- src/typings/bs4.pyi:12:17: subclass-builtin Subclassing `list` can be error prone, use `collections.UserList` instead

home-assistant/core (+0 -1 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --no-fix --output-format concise --preview

- homeassistant/util/hass_dict.pyi:31:16: subclass-builtin Subclassing `dict` can be error prone, use `collections.UserDict` instead

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
subclass-builtin	2	0	2	0	0