test_logging: Fix BytesWarning in SysLogHandlerTest (GH-93920)

(cherry picked from commit 538f289)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: miss-islington

Lib/test/test_logging.py

@@ -1919,7 +1919,7 @@ def setUp(self):
             self.sl_hdlr = hcls((server.server_address[0], server.port))
         else:
             self.sl_hdlr = hcls(server.server_address)
-        self.log_output = ''
+        self.log_output = b''
         self.root_logger.removeHandler(self.root_logger.handlers[0])
         self.root_logger.addHandler(self.sl_hdlr)
         self.handled = threading.Event()