Improve AST safety check

[hauntsaninja]

Fixes #4288, regressed by #4270

[github-actions]

diff-shades reports zero changes comparing this PR (22f2570) to main (c9d2635).

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What is this? | Workflow run | diff-shades documentation

[JelleZijlstra]

I feel ideally we should only reformat real docstrings, but this still preserves the invariant that we only change strings that aren't executed, so this is fine.

Thanks!