Werkzeug safe_join not safe on Windows
On Python < 3.11 on Windows, `os.path.isabs()` does not catch UNC paths like `//server/share`. Werkzeug's `safe_join()` relies on this check, and so can produce a path that is not safe, potentially allowing unintended access to data. Applications using Python >= 3.11, or not using Windows, are not vulnerable.