bpo-33789, test_asyncio: Hide PendingDeprecationWarning (GH-7461)

Hide PendingDeprecationWarning in test__register_task_3().

Author: vstinner

Lib/test/test_asyncio/test_tasks.py

@@ -2613,7 +2613,8 @@ def done(self):
         self.assertEqual(asyncio.all_tasks(loop), set())
         self._register_task(task)
         self.assertEqual(asyncio.all_tasks(loop), set())
-        self.assertEqual(asyncio.Task.all_tasks(loop), {task})
+        with self.assertWarns(PendingDeprecationWarning):
+            self.assertEqual(asyncio.Task.all_tasks(loop), {task})
         self._unregister_task(task)
 
     def test__enter_task(self):