Gremlin-Python only aggregates to list when using GraphSON3

Gremlin-Python fails to correctly extract the aggregate_to value from the Gremlin Server response when using GraphSON3. I believe this is the source of the test failures that were temporarily fixed by using the v2.0 message serializer in this commit.

In general, it seems this value is a bit buried in the new response message format:

{'requestId': 'b2a51ad2-bca6-4493-9115-a57a92e477c7',
 'result': {'data': {'@type': 'g:List',
   '@value': [{'@type': 'g:Map',
     '@value': ['ripple', {'@type': 'g:Int64', '@value': 1}]},
    {'@type': 'g:Map', '@value': ['lop', {'@type': 'g:Int64', '@value': 3}]}]},
  'meta': {'@type': 'g:Map',
   '@value': ['aggregateTo', 'map', 'sideEffectKey', 'm']}},
 'status': {'attributes': {'@type': 'g:Map', '@value': []},
  'code': 200,
  'message': ''}}

To access it, you have to do something like:

message['result']['meta']['@value'][1]

Which throws an error when no metadata is present. This seems a bit clumsy. I haven't looked at how the Java driver deals with it, maybe there is a better way to parse this.