[issue23085] update internal libffi copy to 3.1

[Matthias Klose]

Matthias Klose added the comment:

some issues:

 - the local change for Windows still needs upstream forwarding.
   Steve, any progress on this?

 - the libffi.diff is not updated (including Steve's changes)

 - this should be applied to 2.7 as well.

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nosy: +steve.dower, tim.golden, zach.ware

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<http://bugs.python.org/issue23085>
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