Issue #21090: io.FileIO.readall() does not ignore I/O errors anymore. Before,
it ignored I/O errors if at least the first C call read() succeed.
asyncio: sync with Tulip

* _UnixSubprocessTransport: fix file mode of stdin. Open stdin in write mode,
  not in read mode
* Examples: close the event loop at exit
* More reliable CoroWrapper.__del__. If the constructor is interrupted by
  KeyboardInterrupt or the coroutine objet is destroyed lately, some the
  _source_traceback attribute doesn't exist anymore.
* repr(Task): include also the future the task is waiting for
Fix guidance for subclassing collections.Set()