Fix a crash in preview style with assert + parenthesized string.

[yilei]

Description

The bug is in the get_leaves_inside_matching_brackets on the third line below:

assert xxxxxxxxx.xxxxxxxxx.xxxxxxxxx(
    xxxxxxxxx
).xxxxxxxxxxxxxxxxxx(), (
    "xxx {xxxxxxxxx} xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
)

Including the invisible paren, third line is ).xxxxxxxxxxxxxxxxxx()), (, that it has a matched pair then an unmatched closing paren afterwards. This PR ensures the returned leaves are actually matched.

Fixes #3414.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[yilei]

diff-shades failed (unrelatedly) on mypyc compiling black/__init__.py? https://github.com/psf/black/actions/runs/3663181139/jobs/6192793818#step:6:56

[JelleZijlstra]

Thanks for the fix! #3416 should fix the mypyc issue, let me update the branch for this PR.

[github-actions]

diff-shades reports zero changes comparing this PR (04bb8d2) to main (1f7f6de).

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