test_atexit: fix code saving/restoring stdout and stderr

That's why I prefer a single instruction per line :-)
diff --git a/Lib/test/test_atexit.py b/Lib/test/test_atexit.py
index 9c7e109..5200af7 100644
--- a/Lib/test/test_atexit.py
+++ b/Lib/test/test_atexit.py
@@ -25,8 +25,9 @@
 
 class TestCase(unittest.TestCase):
     def setUp(self):
+        self.save_stdout = sys.stdout
+        self.save_stderr = sys.stderr
         self.stream = io.StringIO()
-        self.save_stdout, self.save_stderr = sys.stderr, sys.stdout
         sys.stdout = sys.stderr = self.stream
         atexit._clear()