Is graphql-relay actually a requirement if relay is not going to be used?

I was hoping that `graphql-relay` could be removed as a base requirement in setup.py if it isn't actually required for people who have no need for relay.

Perhaps a specific install option like `pip install graphene[relay]` could be added for those that actually require it.

Obviously this would be a breaking change so it makes sense to do it before 3.0 is released if it will be done.

Start with setup.py and inspect how graphql-relay is declared and used by the package. Determine whether it can be moved to a relay-specific install option, then verify that a non-relay installation works and that relay users can still install the dependency; account for the proposed breaking change before 3.0.

Written by the indexing model from the issue text.