Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/lib-tk/FileDialog.py b/Lib/lib-tk/FileDialog.py
index 323dc29..5e848da 100644
--- a/Lib/lib-tk/FileDialog.py
+++ b/Lib/lib-tk/FileDialog.py
@@ -244,7 +244,7 @@
                 return
             d = Dialog(self.top,
                        title="Overwrite Existing File Question",
-                       text="Overwrite existing file %s?" % `file`,
+                       text="Overwrite existing file %r?" % (file,),
                        bitmap='questhead',
                        default=1,
                        strings=("Yes", "Cancel"))