• New python_platform configuration setting for Red Knot
• Infer Literal["linux"] etc for sys.platform if config option is set accordingly
• Infer str if config option is not set or set to "all" (https://github.com/python/typeshed/blob/ea368c72696afba1eb4c12653123edd764c800bf/stdlib/sys/__init__.pyi#L49). Note: this could be LiteralString, but it should be changed in typeshed instead of patching it in Red Knot.

(this is already implemented, this is mostly a marker that no one else starts working on this)