[red-knot] visit with-item vars even if not a Name

[carljm]

This fixes the last panic on checking pandas.

(Match statement became an if let because clippy decided it wanted that once I added the additional line in the else case?)

[AlexWaygood]

I think we should see if we can setup a test that runs on all the test Python snippets we have for the parser as well. We have very extensive parser coverage for with statements; it would have caught this :-)

[github-actions]

ruff-ecosystem results

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