[flynt] Use raw triple quotes for joined raw strings with newlines

[TaKO8Ki]

Summary

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.

[github-actions]

ruff-ecosystem results

Linter (stable)

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Linter (preview)

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[ntBre]

Thanks! I think we can simplify a bit, and there are also some edge cases we still need to handle.

[ntBre]

I think we could use a combination of StringFlags::quote_str and Quote::opposite to simplify this section a bit.

Should we add a test case (or a few test cases) for this behavior?

I also think we may need to check for both ''' and """ in every string. We could end up with a pathological case like this:

'\n'.join([r"raw string", '<""">', "<'''>"])

which we can't join into a single string with either delimiter. I think if we have a raw string, and the joined string contains our attempted triple quotes, we'll just have to bail out without a fix.

CPython escapes some of the quotes in this case:

>>> '\n'.join([r"raw string", '<""">', "<'''>"])
'raw string\n<""">\n<\'\'\'>'

I think we could also do that with our UnicodeEscape type, but I'd be fine with, or even prefer, avoiding the fix in that case.

[TaKO8Ki]

Thank you for the suggestion. I have addressed your comment.