[py3] Avoided relying on 2.x-only internals

in LiveServerTestCase.

django/test/testcases.py

@@ -917,7 +917,10 @@ class QuietWSGIRequestHandler(WSGIRequestHandler):
         pass
 
 
-class _ImprovedEvent(threading._Event):
+if sys.version_info >= (2, 6, 0):
+    _ImprovedEvent = threading._Event
+else:
+    class _ImprovedEvent(threading._Event):
         """
         Does the same as `threading.Event` except it overrides the wait() method
         with some code borrowed from Python 2.7 to return the set state of the