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:
To access it, you have to do something like:
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.