bpo-43723: Fix deprecation error caused by thread.setDaemon() (GH-25361)

diff --git a/Lib/test/test_telnetlib.py b/Lib/test/test_telnetlib.py
index 8e36051..41c4fcd 100644
--- a/Lib/test/test_telnetlib.py
+++ b/Lib/test/test_telnetlib.py
@@ -29,7 +29,7 @@ def setUp(self):
         self.sock.settimeout(60)  # Safety net. Look issue 11812
         self.port = socket_helper.bind_port(self.sock)
         self.thread = threading.Thread(target=server, args=(self.evt,self.sock))
-        self.thread.setDaemon(True)
+        self.thread.daemon = True
         self.thread.start()
         self.evt.wait()