Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/distutils/dist.py b/Lib/distutils/dist.py
index d313e7d..f63ea97 100644
--- a/Lib/distutils/dist.py
+++ b/Lib/distutils/dist.py
@@ -525,9 +525,9 @@
                         func()
                     else:
                         raise DistutilsClassError(
-                            "invalid help function %s for help option '%s': "
+                            "invalid help function %r for help option '%s': "
                             "must be a callable object (function, etc.)"
-                            % (`func`, help_option))
+                            % (func, help_option))
 
             if help_option_found:
                 return