Python: Add `Str` class

[tausbn]

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This makes the AST viewer (which annotates string constant nodes as Str, and not StrConst) a bit more consistent.

Fixes #9833.

It might be even better to instead flip the inheritance and make Str the lowest class in the hierarchy. That way, it would have access to all of the methods on StrConst (and indeed, we might consider just deprecating that class).

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This is a very minor change, so I don't think a change note is required.