@haypo, I checked available info online and couldn't find any reference to byte file-paths were deprecated since Python3.2.

On the contrary, the 3.2 release notes [0] state:

 "countless fixes regarding bytes/string issues; among them full support for a bytes environment (filenames, environment variables)"

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Also docs for open 3.2 [1] and 3.5 [2] say that byte filenames are supported with no mention of deprecation.

Since this is already working properly in 3.5 for other systems besides ms-windows, and worked in 3.4x.
Dropping support on a single platform seems a rather problematic regression.

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[0]: https://www.python.org/download/releases/3.2/
[1]: https://docs.python.org/3.2/library/functions.html#open
[2]: https://docs.python.org/3.5/library/functions.html#open