[issue18373] implement sys.get/setbyteswarningflag()

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka at gmail.com>:


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components: +Interpreter Core
stage:  -> patch review
type:  -> enhancement
versions: +Python 3.5 -Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18373>
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