Ariadne

- Refactored `ariadne.asgi.GraphQL` to use strategy pattern for handling HTTP and WebSockets.
- Updated `load_schema_from_path` to also support `.gql` and `.graphqls` files.
- Added support for `starlette` 0.20.

- Fix performance regression in `make_federated_schema`.

- Updated `graphql-core` requirement to 3.2.0.
- Bumped `starlette` supported versions to 0.18 and 0.19.
- Drop Python 3.6 support.
- Added basic support for `OPTIONS` HTTP request.
- Refactor `ariadne.asgi.GraphQL` to make it easier to customize JSON response.
- Added `trace_default_resolver` to `ApolloTracingExtension` that enables tracing for default resolvers.
- Fixed `make_federated_schema` error when custom directive in schema has description.
- Moved `set_default_enum_values_on_schema`, `validate_schema_enum_values` and `type_implements_interface` to public API.
- Changed `graphql_sync` to use `execute_sync` instead of `execute`.
- Added `on_operation` hook to `ariadne.asgi.GraphQL` that's called when individual subscription operation is started.
- Added `on_complete` hook to `ariadne.asgi.GraphQL` that's called when individual subscription operation is completed.
- Updated `on_disconnect` hook so its called in Webhook handler's `finally` clause, making it called in more situations.
- Marked `Extension`, `ExtensionSync` and `SchemaBindable` protocols as `runtime_checkable`.
- Renamed `parent` to `obj` in `ApolloTracing` and `OpenTracing` extensions so arg name won't cause conflict when custom resolver has `parent` arg.

- Remove `ariadne.contrib.django` from release.
- Lock `GraphQL-core` requirement at <3.2.

- Added `on_connect` and `on_disconnect` options to `ariadne.asgi.GraphQL`, enabling developers to run additional initialization and cleanup for websocket connections.
- Updated Starlette dependency to 0.17.1.
- Added support for multiple keys for GraphQL federations.
- Made `Query` type optional in federated schemas.
- Updated default resolvers to test for `Mapping` instead of `dict`.
- Removed `ariadne.contrib.django`. (Use [ariadne_django](https://github.com/reset-button/ariadne_django) instead).
- Updated query cost validator to handle optional variables.

- Updated `graphql-core` requirement to 3.1.3.
- Added support for Python 3.9.
- Added support for using nested variables as cost multipliers in the query price validator.
- `None` is now correctly returned instead of `{"__typename": typename}` within federation.
- Fixed some surprising behaviors in `convert_kwargs_to_snake_case` and `snake_case_fallback_resolvers`.