'Get a datetime object from a dict, return None if it wan\'t found.
This is equivalent to calling::
cls._strptime(data[attribute]) if attribute in data else None
:param dict data: dictionary used to put together the model
:param str attribute: key of the attribute
:returns: timezone-aware datetime object
:rtype: datetime'

@classmethod
def _strptime_attribute(cls, data, attribute):

    result = cls._get_attribute(data, attribute)
    if result:
        return cls._strptime(result)
    return result
