Fix retry logic to prevent setting exception if future is already done

[ofekby]

This pull request makes a small but important change to the retry logic in the __retry method of the src/aioice/stun.py file. The change ensures that the TransactionTimeout exception is not redundantly set if the future is already completed.

• src/aioice/stun.py: Modified the condition in the __retry method to check if the future (self.__future) is not already done before setting the TransactionTimeout exception. This prevents unnecessary operations and potential errors.

We experience this case in production here is the stacktrace:

Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 253, in uvloop.loop.TimerHandle._run
  File "[masked]/aioice/stun.py", line 330, in __retry
    self.__future.set_exception(TransactionTimeout())
asyncio.exceptions.InvalidStateError: invalid state