[issue23668] Support os.ftruncate on Windows

[Steve Dower]

Steve Dower added the comment:

Sorry, _Py_open requires a double encoding (wchar->char->wchar), which is why I'm not going to use it.

_Py_wfopen takes a mode string rather than _O_* flags, and so implicitly includes _O_CREAT.

Guessing we should add _Py_wopen?

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