asyncio: sync with Tulip

- 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()

Lib/test/test_asyncio/test_tasks.py

@@ -1621,6 +1621,7 @@ class TaskTests(test_utils.TestCase):
                          (__file__,
                           lineno,
                           'test_task_source_traceback'))
+        self.loop.run_until_complete(task)
 
 
 class GatherTestsBase: