Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/lib-tk/Tix.py b/Lib/lib-tk/Tix.py
index 99731cd..9114538 100755
--- a/Lib/lib-tk/Tix.py
+++ b/Lib/lib-tk/Tix.py
@@ -374,9 +374,9 @@
         if option == '':
             return
         elif not isinstance(option, StringType):
-            option = `option`
+            option = repr(option)
         if not isinstance(value, StringType):
-            value = `value`
+            value = repr(value)
         names = self._subwidget_names()
         for name in names:
             self.tk.call(name, 'configure', '-' + option, value)