Changelog

• 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.