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b85be629 - [red knot] Minor follow-up tasks regarding singleton types (#13769)

[red knot] Minor follow-up tasks regarding singleton types (#13769) ## Summary - Do not treat empty tuples as singletons after discussion [1] - Improve comment regarding intersection types - Resolve unnecessary TODO in Markdown test [1] https://discuss.python.org/t/should-we-specify-in-the-language-reference-that-the-empty-tuple-is-a-singleton/67957 ## Test Plan —