Django-channels-graphql-ws

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.

- DjangoChannelsGraphqlWs has migrated to the recent versions of Django,
Channels, and Graphene. All other Python dependencies updated.
- Server outputs a warning to the log when operation/resolver takes
longer than specified timeout, which is one second by default. The
settings `GraphqlWsConsumer.warn_operation_timeout` and
`GraphqlWsConsumer.warn_resolver_timeout` allow to tune the timeout or
even disable the warning at all.
- If exception raises from the resolver a response now contains a field
"extensions.code" with a class name of the exception.
- Added support for async resolvers and middlewares.
- WARNING: This release is not backward compatible with previous ones!
The main cause is a major update of Django, Channels, and Graphene,
but there are some introduced by the library itself. In particular:
- Context lifetime and content have changed. See README.md for
details.

- Minor fix in logging.