Add support for DSE Graph in the Python driver API.
PR presently just wraps a Session in a GraphSession. We're still discussing the merits of wrapping vs extending. Some discussion here: https://github.com/datastax/java-driver-dse/pull/3
Many of the points for wrapping seem to be java-specific. I wanted to put this up for review to start a discussion focused on Python. I was leaning toward adding graph execute methods to the specialized session created for the geo ticket.
Looks good to me @ccae3183ea69b4e8cd8beebe91e88393970a6ee2. "GraphBoundStatement" is mentioned in one of the comments but not implemented.
I like the compostability of wrapping versus extending. Wrapping allows a single session it to be used in multiple ways.
Extending still allows it to be used for cql and graph. I won't expound too much more as there are ongoing debates elsewhere.
For now, merging with wrapped approach.
I'm moving this to done, with the understanding that the tests are not running reliably in Jenkins, but from a functional standpoint it's tested.