restore _set_event_loop_policy(None)

Author: graingert

Lib/test/test_asyncio/test_taskgroups.py

@@ -13,6 +13,11 @@
 
 from test.test_asyncio.utils import await_without_task
 
+# To prevent a warning "test altered the execution environment"
+def tearDownModule():
+    asyncio._set_event_loop_policy(None)
+
+
 class MyExc(Exception):
     pass