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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Fixes#19887
- flynt(FLY002): When joining only string constants, upgrade raw
single-quoted strings to raw triple-quoted if the resulting
content contains a newline.
- Choose a safe triple-quote delimiter by switching to the opposite
quote style if the preferred triple appears inside the
content.
- Update FLY002 snapshot to include the `\n'.join([r'line1','line2'])`
case.
## Test Plan
I've added one test case to FLY002.py.
<!-- How was it tested? -->
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Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>
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