Closes #21886, #21447: Fix a race condition in asyncio when setting the result
of a Future with call_soon(). Add an helper, a private method, to set the
result only if the future was not cancelled.
Closes #21921: Fix ResourceWarning in the asyncio examples: close the event
loop at exit. Patch written by Vajrasky Kok (I modified also the "hello world"
example using a coroutine).