[pylint] - restrict iteration-over-set to only work on sets of literals (PLC0208)

[diceroll123]

Summary

Restricts iteration-over-set to only work on sets of literals. There are valid use cases for iterating over a set, for de-duplication purposes.

Fixes #11694

Test Plan

cargo test

[diceroll123]

I'm on the fence about still emitting a diagnostic without a fix even for "intentional" use of a set iteration, please advise. 😄

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Limiting the rule to only case where we can prove that the values are unique seems reasonable to me. Wdyt @AlexWaygood

But I suggest that we use ComparableExpr to ensure that the literals are unique.

[MichaReiser]

Technically, this also applies to literals. E.g. it's possible that someone wrote set(1, 2, 2) where fixing it to [1, 2, 2] would result in a semantic change.

That's why I'm inclined that we should instead use ComparableExpr to test if no two set entries are identical. See e.g.

ruff/crates/ruff_linter/src/rules/pyflakes/rules/repeated_keys.rs

Line 141 in aa0db33

let comparable_key = ComparableExpr::from(key);