After some tests on Windows, I realized that my patch is not enough to be fully unicode compliant (on Windows). Some functions are still using PyUnicode_DecodeFSDefault() or PyUnicode_EncodeFSDefault(). Until all functions are patched to use unicode strings, Python3 will not be fully unicode compliant *on Windows*. The problem is specific to Windows, because Python uses mbcs codec which doesn't support surrogateescape error handler.

I think that this patch is already huge and complex, and it will be difficult to fix all issues at the same time. This patch does improve the situation: with the patch, Python is fully unicode compliant (except on Windows), and it fixes at least one issue on Windows (#8988, it now uses the right encoding).