Make sure Black doesn't crash when fmt:off is used before a closing paren

[yilei]

Description

When a # fmt: off is used before a closing paren, it's invalid as Black demands that the # fmt: on/off pairs must be on the same level of indentation and in the same block. So this PR choses to ignore it. Previously it would crash as it ends up with one extra closing paren.

• Fix INTERNAL ERROR: Black produced invalid code on pass 1: unmatched ')' #2478
• Fix Black crashes with a single # fmt: off before a dict's closing bracket #3458

(Also refactored the convert_one_fmt_off_pair function a little bit.)

Checklist - did you ...

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• Add new / update outdated documentation?

[github-actions]

diff-shades reports zero changes comparing this PR (a732cc2) to main (b9c6323).

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[JelleZijlstra]

Thank you!