bpo-40165: Suppress stderr when checking if test_stty_match should be skipped (GH-19325)

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 362ba9e..0db7d30 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -3485,7 +3485,11 @@
         should work too.
         """
         try:
-            size = subprocess.check_output(['stty', 'size']).decode().split()
+            size = (
+                subprocess.check_output(
+                    ["stty", "size"], stderr=subprocess.DEVNULL, text=True
+                ).split()
+            )
         except (FileNotFoundError, subprocess.CalledProcessError,
                 PermissionError):
             self.skipTest("stty invocation failed")