pythongh-131178: Fix test_unknown_flag for platform CLI

Author: donbarbos

Lib/test/test_platform.py

@@ -763,12 +763,12 @@ def invoke_platform(self, *flags):
         return output.getvalue()
 
     def test_unknown_flag(self):
+        output = io.StringIO()
         with self.assertRaises(SystemExit):
-            output = io.StringIO()
             # suppress argparse error message
             with contextlib.redirect_stderr(output):
                 _ = self.invoke_platform('--unknown')
-            self.assertStartsWith(output, "usage: ")
+        self.assertStartsWith(output.getvalue(), "usage: ")
 
     def test_invocation(self):
         flags = (