Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Tools/scripts/untabify.py b/Tools/scripts/untabify.py
index 17e9166..9bdf235 100755
--- a/Tools/scripts/untabify.py
+++ b/Tools/scripts/untabify.py
@@ -29,7 +29,7 @@
         text = f.read()
         f.close()
     except IOError, msg:
-        print "%s: I/O error: %s" % (`filename`, str(msg))
+        print "%r: I/O error: %s" % (filename, msg)
         return
     newtext = text.expandtabs(tabsize)
     if newtext == text: