"Invalid" check for Mappings

[romuald]

Small issue with dict-like mappings

In the Mapping _validate method

Colander is checking if there is an "items" attribute

However python (2) checks if there is a keys attribute instead : https://github.com/python/cpython/blob/2.7/Objects/dictobject.c#L1475

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Note that fixing by replacing items by keys this might break some existing validation. I'm just submitting this as a possible issue (maybe simply try to use dict(value)?)