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mbcs encoding replace non encodable characters (loose information) and doesn't support surrogateescape error handler. It ignores the error handler argument: see #850997, and tarfile now uses surrogateescape error handler by default (#8390). This encoding is just horrible for unicode support :-)
Since Windows native API use unicode character (UTF-16), I think that it would be better to use utf-8 for the default encoding on Windows. utf-8 is able to encode and decode the full Unicode charset and supports all error handlers (especially surrogateescape).
Attached patch sets the default encoding to utf-8 on Windows, and removes the test ENCODING is None because sys.getfilesystemencoding() cannot be None anymore (in 3.2 only, it's a recent change: #8610).