Ignore asyncio.set_event_loop_policy() deprecation warning in test

Ignore `asyncio.set_event_loop_policy()` deprecation warning when called
directly in test.  I have scoped the ignore to the test call rather than
doing that globally in `pytest.ini`, so that it will be caught if
introduced elsewhere in the code.

Author: mgorny

tests/test_async_py3.py

@@ -1745,6 +1745,8 @@ async def f():
         pass
     finally:
         loop.close()
-        asyncio.set_event_loop_policy(None)
+        with warnings.catch_warnings():
+            warnings.simplefilter('ignore', DeprecationWarning)
+            asyncio.set_event_loop_policy(None)
 
     assert buffer == [1, 2, 'MyException']