[issue19165] Change formatter warning to DeprecationWarning in 3.5

[R. David Murray]

New submission from R. David Murray:

You two may know what this is about, but I have no clue :)  A few more details would help if someone wants to try their hand at a patch.

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nosy: +r.david.murray

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19165>
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