[red-knot] Improve ergonomics for the PySlice trait

[AlexWaygood]

Summary

Just a minor change to improve the ergonomics of the PySlice trait. By implementing PySlice for [T] rather than &[T], we can call it on Vec<T> and Box<[T]> without having to have intermediate calls to .as_slice() or .as_ref(). It can still be called on &[T] and &Box<[T]>, because the py_slice trait method takes &self.

Test Plan

cargo test -p red_knot_python_semantic

[carljm]

Nice!

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[sharkdp]

Thank you.