Correctly handle trailing commas that are inside a line's leading non-nested parens

[yilei]

Description

Correctly handle trailing commas that are inside a line's leading non-nested parens.

• Fixes Only the last trailing comma in a statement is magic #1671
• Fixes Trailing commas not removed when dict/function call collapsed into single line #3229

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• Add new / update outdated documentation?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 results comparing this PR (6819dfc) to main (ffaaf48). The full diff is available in the logs under the "Generate HTML diff report" step.

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│ 2 projects & 7 files changed / 39 changes [+30/-9]     │
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│ ... out of 2 328 882 lines, 10 723 files & 23 projects │
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Differences found.

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[yilei]

Alright, this PR is ready for review. It should unblock #3368.

My initial implementation accidentally fixed the trailing comma issue in #2052, but it had other issues and was wrong so I removed that reference. I'll take another look once this PR is merged.

[JelleZijlstra]

Looks good!