v0.240.0 updates Schema.subscribe โ€™s signature

In order to support schema extensions in subscriptions and errors that can be raised before the execution of the subscription, we had to update the signature of Schema.subscribe .

Previously it was: 

async def subscribe(
    self,
    query: str,
    variable_values: Optional[Dict[str, Any]] = None,
    context_value: Optional[Any] = None,
    root_value: Optional[Any] = None,
    operation_name: Optional[str] = None,
) -> Union[AsyncIterator[GraphQLExecutionResult], GraphQLExecutionResult]:

Now it is: 

async def subscribe(
    self,
    query: Optional[str],
    variable_values: Optional[Dict[str, Any]] = None,
    context_value: Optional[Any] = None,
    root_value: Optional[Any] = None,
    operation_name: Optional[str] = None,
) -> Union[AsyncGenerator[ExecutionResult, None], PreExecutionError]: