[red-knot] Allow type[] to be subscripted

[AlexWaygood]

Fixed a TODO by adding another TODO. It's the red-knot way!

Summary

builtins.type can be subscripted at runtime on Python 3.9+, even though it has no __class_getitem__ method and its metaclass (which is... itself) has no __getitem__ method. The special case is hardcoded directly into PyObject_GetItem in CPython. We just have to replicate the special case in our semantic model.

This will fail at runtime on Python <3.9. However, there's a bunch of outstanding questions (detailed in the TODO comment I added) regarding how we deal with subscriptions of other generic types on lower Python versions. Since we want to avoid too many false positives for now, I haven't tried to address this; I've just made type subscriptable on all Python versions.

Test Plan

cargo test -p red_knot_python_semantic --lib

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.