[issue19543] Add -3 warnings for codec convenience method changes

[Roundup Robot]

Roundup Robot added the comment:

New changeset cf6e782a7f94 by Serhiy Storchaka in branch '2.7':
Issue #19543: Emit deprecation warning for known non-text encodings.
https://hg.python.org/cpython/rev/cf6e782a7f94

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nosy: +python-dev

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<http://bugs.python.org/issue19543>
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