Second beta release for Graphene v3.
Changes: https://github.com/graphql-python/graphene/compare/v3.0.0b0...v3.0.0b1
Highlights:
* Added subscription support! https://github.com/graphql-python/graphene/pull/1107 Thanks to rob-blackbourn for this fantasic contribution
* Remove subclass polyfill 1156
* The `source` attribute now uses the default resolver function 1155