Django-channels-graphql-ws

- Supported the most recent WebSocket sub-protocol
`graphql-transport-ws` used by Apollo. See the specification:
https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md.

- GraphQL parsing and message serialization now perform concurrently
by `sync_to_async(...,thread_sensitive=False)`.

WARNING: Release contains backward incompatible changes!

- To suppress/drop subscription notification just return `None` from the
subscription resolver. Previously it was necessary to return special
`SKIP` object which is no longer the case..
- Python 3.8 compatibility brought back. Tests pass OK.

- `GraphqlWsConsumer.warn_resolver_timeout` removed to avoid mess with
user specified middlewares. This functionality can easily be
implemented on the library user level by creating a designated
middleware.
- `GraphqlWsConsumer.middleware` accepts an instance of
`graphql.MiddlewareManager` or the list of functions. Same as the
argument `middleware` of `graphql.execute` method.
- Fixed broken example.

- Invoke synchronous resolvers in the main thread with eventloop. So
there is no difference in this aspect with async resolvers. This
corresponds to the behavior of the
[`graphql-core`](https://github.com/graphql-python/graphql-core)
library.
- Added example of middleware which offloads synchronous resolvers to
the threadpool.
- Fixed bug with GraphQL WrappingTypes like GraphQLNonNull causing
exceptions when used as subscription field.
- Fixed broken example.

Broken support of previous Python version brought back.