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)

The issue names no repository files or tests; start by reviewing the attached patch, graphql-js #1169, and the Relay 1.6 release notes. The work is complete only after the project settles where compatibility belongs and supports the new syntax while handling the legacy syntax according to that decision.

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