Fix incorrectly setting cancelled futures

[Hellzed]

Hi there,

I'm running aiodataloader in production, not in a GraphQL context, but in a concurrent message consumer. Some messages consumed in the same event loop iteration may refer to the same data, and an uncached DataLoader helps with the IO load.

When the connection to the message broker drops, the current message handling tasks will get cancelled.

This leads to this typical log message:

ERROR    asyncio:base_events.py:1820 Task exception was never retrieved
future: <Task finished name='Task-65' coro=<dispatch_queue_batch() done, defined at /home/loris/Code/aiodataloader-next/aiodataloader/__init__.py:247> exception=InvalidStateError('invalid state')>
Traceback (most recent call last):
  File "/home/loris/Code/aiodataloader-next/aiodataloader/__init__.py", line 300, in dispatch_queue_batch
    ql.future.set_result(value)
asyncio.exceptions.InvalidStateError: invalid state

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/loris/Code/aiodataloader-next/aiodataloader/__init__.py", line 303, in dispatch_queue_batch
    return failed_dispatch(loader, queue, e)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/loris/Code/aiodataloader-next/aiodataloader/__init__.py", line 315, in failed_dispatch
    ql.future.set_exception(error)
asyncio.exceptions.InvalidStateError: invalid state

It turns out aiodataloader is not checking if an asyncio.Future is not already cancelled before attempting to set it, which is against asyncio recommended practice.

This PR enforces this check for all asyncio.Future set by aiodataloader.

[Hellzed]

@syrusakbary There seems to be an issue with branch checks, I'm unsure what's he next step.

[markedwards]

This generally looks good. You have some type errors in test_aiodataloader.py you need to fix. I will do a PR to sort out the failing test runs and add a test run for newer python versions.

[Hellzed]

I've just fixed the type errors, let me know if there's anything else I can help with.

[markedwards]

I have updated GitHub actions, please update and re-trigger CI.