Use private _set_event_loop_policy API to avoid DeprecationWarning

Author: heliang666s

Lib/test/test_asyncio/test_threads.py

@@ -3,16 +3,13 @@
 import asyncio
 import unittest
 import functools
-import warnings
 
 from contextvars import ContextVar
 from unittest import mock
 
 
 def tearDownModule():
-    with warnings.catch_warnings():
-        warnings.simplefilter("ignore", DeprecationWarning)
-        asyncio.set_event_loop_policy(None)
+    asyncio._set_event_loop_policy(None)
 
 
 class ToThreadTests(unittest.IsolatedAsyncioTestCase):