[issue23668] Support os.ftruncate on Windows

[Steve Dower]

Steve Dower added the comment:

I responded to Victor's suggestion about _Py_open instead of _open, but on rereading I see that it also handles EINTR. 

AFAICT (from https://msdn.microsoft.com/en-us/library/5814770t.aspx, mainly), Windows isn't ever going to return EINTR from the CRT. I don't especially want to have to modify _Py_open to take another parameter to suppress creating a new file, but if someone else does then I'm happy to use it.

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<http://bugs.python.org/issue23668>
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