Python: Add `Str` class
Python: Add `Str` class

This makes the AST viewer (which annotates string constant nodes as
`Str`) a bit more consistent.

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql