Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Mac/Tools/IDE/PyDebugger.py b/Mac/Tools/IDE/PyDebugger.py
index 18a089e..51ba753 100644
--- a/Mac/Tools/IDE/PyDebugger.py
+++ b/Mac/Tools/IDE/PyDebugger.py
@@ -511,7 +511,7 @@
 					return self.dispatch_return(frame, arg)
 				if event == 'exception':
 					return self.dispatch_exception(frame, arg)
-				print 'bdb.Bdb.dispatch: unknown debugging event:', `event`
+				print 'bdb.Bdb.dispatch: unknown debugging event:', repr(event)
 				return self.trace_dispatch
 			finally:
 				if hasattr(MacOS, 'EnableAppswitch'):