- Sort imports
- Simplify/optimize iscoroutine(). Inline inspect.isgenerator(obj): replace it
  with isinstance(obj, types.GeneratorType)
- CoroWrapper: check at runtime if Python has the yield-from bug #21209.  If
  Python has the bug, check if CoroWrapper.send() was called by yield-from to
  decide if parameters must be unpacked or not.
- Fix "Task was destroyed but it is pending!" warning in
  test_task_source_traceback()