client.submitAsync returns None value

In gremlin-python driver versions 3.5.2 and 3.5.3, the client.submit_async() function was added and the client.submitAsync() function was changed to call into it and warn about future deprecation.

The line in submitAsync which invokes the new function submit_async does not return the future back to the caller, resulting in the caller receiving a None object.

https://github.com/apache/tinkerpop/blob/8b30b63d00630d3b6c6bd170e8168aed7c69abff/gremlin-python/src/main/python/gremlin_python/driver/client.py#L152

This in turn means that an application which upgrades from 3.4.x to latest 3.5.x will break when trying to read result from the response future.

> AttributeError: 'NoneType' object has no attribute 'result'

This issue is fixed in 3.6.x with the addition of an explicit return statement, but has persisted from driver version 3.5.2 to 3.5.3.