Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/idlelib/TreeWidget.py b/Lib/idlelib/TreeWidget.py
index 824bdca..1c9eb2e 100644
--- a/Lib/idlelib/TreeWidget.py
+++ b/Lib/idlelib/TreeWidget.py
@@ -31,7 +31,7 @@
 if os.path.isdir(_icondir):
     ICONDIR = _icondir
 elif not os.path.isdir(ICONDIR):
-    raise RuntimeError, "can't find icon directory (%s)" % `ICONDIR`
+    raise RuntimeError, "can't find icon directory (%r)" % (ICONDIR,)
 
 def listicons(icondir=ICONDIR):
     """Utility to display the available icons."""