Adopt "new-style" schema definition language syntax

[obi1kenobi]

Per graphql/graphql-js#1169

This replaces:

type Foo implements Bar, Baz { field: Type }

With:

type Foo implements Bar & Baz { field: Type }

With no changes to the common case of implementing a single interface.

We may need to update to a more up-to-date GraphQL core library if one exists.

[obi1kenobi]

GraphQL core appears to have a newer version out than we are currently using. Assigning to @pmantica1 to take a look if that will resolve this issue.

[pmantica1]

Updating the GraphQL core version will not fix this issue. The interfaces implemented by an object are still delimited by "," and not "&" as shown in the link below.
https://github.com/graphql-python/graphql-core/blob/master/graphql/utils/schema_printer.py#L131

The interfaces are delimited by "&" though in the GraphQL core-next library as shown below.
https://github.com/graphql-python/graphql-core-next/blob/master/src/graphql/utilities/schema_printer.py#L154

[obi1kenobi]

Ok so this would require dropping Python 2 support, which might be worth it for this. @pmantica1, could you check what it would take to do the migration? I believe there may be some breaking changes between graphql-core and graphql-core-next, so we should put together a migration plan if it's not as simple as changing dependencies.