RuntimeError when subscription connection is reset.

Sometimes when a subscription client has reset their connection, tartiflette-aiohttp will raise a RuntimeError with the message: dictionary changed size during iteration. I've tracked the issue down to the following line:

tartiflette-aiohttp/tartiflette_aiohttp/_subscription_ws_handler.py

Line 231 in 85d349d

for operation_id in connection_context.operations:

Changing this to list(connection_context.operations.keys()) fixed the issue for me locally, not sure if that's the preferred approach though. I don't have a simple test case for this but mutating the dictionary while iterating over it is certainly a bug anyways.