[issue13559] Use sendfile where possible in httplib

[Giampaolo Rodola']

Changes by Giampaolo Rodola' <g.rodola at gmail.com>:


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dependencies: +Add a new socket.sendfile() method
versions: +Python 3.5 -Python 3.3

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13559>
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