Explicitly specify GraphSON V3 serializer when creating Gremlin connection

[michaelnchin]

Is your feature request related to a problem? Please describe.

As of gremlinpython==3.6.x, the Gremlin Python client currently uses GraphSON V3 serializer as the default encoding format when creating a new connection: https://github.com/apache/tinkerpop/blob/42cefe106a3375aab497cbfc477178fa83d08325/gremlin-python/src/main/python/gremlin_python/driver/client.py#L55

In the future, new versions of Gremlin Python (and all other Gremlin clients) are planned to move over to GraphBinary as the new default serializer.

To ensure compatibility, we should consider specifying GraphSON V3 as the desired serializer when creating the gremlin connection here, via the message_serializer argument.