Initialize a KeyValuePair object from a json dictionary.

def _from_dict(cls, _dict):
        """Initialize a KeyValuePair object from a json dictionary."""
        args = {}
        if 'key' in _dict:
            args['key'] = Key._from_dict(_dict.get('key'))
        if 'value' in _dict:
            args['value'] = Value._from_dict(_dict.get('value'))
        return cls(**args)