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

Lib/test/test_asyncio/test_tasks.py

@@ -218,6 +218,17 @@ def notmuch():
                          '<Task pending %s cb=[<Dummy>()]>' % coro)
         self.loop.run_until_complete(t)
 
+    def test_task_repr_wait_for(self):
+        @asyncio.coroutine
+        def wait_for(fut):
+            return (yield from fut)
+
+        fut = asyncio.Future(loop=self.loop)
+        task = asyncio.Task(wait_for(fut), loop=self.loop)
+        test_utils.run_briefly(self.loop)
+        self.assertRegex(repr(task),
+                         '<Task .* wait_for=%s>' % re.escape(repr(fut)))
+
     def test_task_basics(self):
         @asyncio.coroutine
         def outer():