Streamline inheritance for gremlin-python GraphSON serializer classes

This follows up on TINKERPOP-1427 by cleaning up the code used to create the various gremlin-python GraphSON serializers for GraphSON version 2 and 3. Several small changes can improve code style, reduce redundancy, and clarify when custom readers and writers can be passed to the serializer (generic serializer) vs. the GraphSON version specific serializers.