Workaround for aiorun triggering stackoverflow in Python (#88)

Under certain circumstances this line in `aiorun` can trigger the following
Python bug python/cpython#93837

The program will crash with a stack overflow error, probably because the
call to log the task about the be cancelled will call repr on a task that
references itself somehow.

aiorun.py

@@ -294,7 +294,6 @@ async def new_coro():
             # TODO: we don't need access to the coro. We could simply
             # TODO: store the task itself in the weakset.
             if t._coro not in _DO_NOT_CANCEL_COROS:
-                logger.debug("Cancelling task: %s", t)
                 t.cancel()
 
     async def wait_for_cancelled_tasks(timeout):