Add set_event_loop_policy to backports

Since it's been deprecated in newer Python versions

Author: cjw296

Diff for: mock/backports.py

@@ -90,3 +90,9 @@ def run(self, result=None):
                 return super().run(result)
             finally:
                 self._tearDownAsyncioLoop()
+
+
+try:
+    from asyncio import _set_event_loop_policy as set_event_loop_policy
+except ImportError:
+    from asyncio import set_event_loop_policy

Diff for: mock/tests/testasync.py

@@ -8,7 +8,7 @@
 
 from mock import (ANY, call, AsyncMock, patch, MagicMock, Mock,
                   create_autospec, sentinel, seal)
-from mock.backports import IsolatedAsyncioTestCase, iscoroutinefunction
+from mock.backports import IsolatedAsyncioTestCase, iscoroutinefunction, set_event_loop_policy
 from mock.mock import _CallList
 
 
@@ -25,7 +25,7 @@ def run(main):
 
 
 def tearDownModule():
-    asyncio._set_event_loop_policy(None)
+    set_event_loop_policy(None)
 
 
 class AsyncClass: