Support new multiple inheritance syntax

`graphql-js 0.13` introduced a new multiple inheritance syntax, see [#1169](https://github.com/graphql/graphql-js/pull/1169). `relay 1.6` will no longer support the old syntax, see [release notes](https://github.com/facebook/relay/releases).

Implementing the new and dropping support for the old syntax is straight forward (patch attached) but would be a breaking change. A switch like `useLegacySDLImplementsInterfaces` sounds like a better idea but I am not sure what would be the best place for that in `graphql-python` (`GraphQLSchema`?).
[0002-new-multiple-inheritance-syntax.txt](https://github.com/graphql-python/graphql-core/files/2001005/0002-new-multiple-inheritance-syntax.txt)