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6b9f3d7d - [red-knot] Import `LiteralString`/`Never` from `typing_extensions` (#14817)

[red-knot] Import `LiteralString`/`Never` from `typing_extensions` (#14817) ## Summary `typing.Never` and `typing.LiteralString` are only conditionally exported from `typing` for Python versions 3.11 and later. We run the Markdown tests with the default Python version of 3.9, so here we change the import to `typing_extensions` instead, and add a new test to make sure we'll continue to understand the `typing`-version of these symbols for newer versions. This didn't cause problems so far, as we don't understand `sys.version_info` branches yet. ## Test Plan New Markdown tests to make sure this will continue to work in the future.