[3.9] Fix warning: asyncio.events._event_loop_policy was modified by test_asyncio (GH-31253). (GH-31256)

(cherry picked from commit 012e77eb5c)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Lib/test/test_asyncio/test_sock_lowlevel.py

@@ -10,6 +10,10 @@
 from test.support import socket_helper
 
 
+def tearDownModule():
+    asyncio.set_event_loop_policy(None)
+
+
 class MyProto(asyncio.Protocol):
     connected = None
     done = None