relay Connection.node does not have to subclass Node/ObjectType

[sjhewitt]

https://facebook.github.io/relay/graphql/connections.htm#sec-Node

[coveralls]

Coverage decreased (-0.009%) to 91.146% when pulling 8763424 on sjhewitt:connection-node into f955280 on graphql-python:master.

[coveralls]

Coverage remained the same at 91.155% when pulling a77b279 on sjhewitt:connection-node into f955280 on graphql-python:master.

[coveralls]

Coverage remained the same at 91.155% when pulling a77b279 on sjhewitt:connection-node into f955280 on graphql-python:master.

[syrusakbary]

Good catch! 👍