Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Doc/tools/refcounts.py b/Doc/tools/refcounts.py
index d82def7..ccfc8c6 100644
--- a/Doc/tools/refcounts.py
+++ b/Doc/tools/refcounts.py
@@ -32,7 +32,7 @@
             continue
         parts = line.split(":", 4)
         if len(parts) != 5:
-            raise ValueError("Not enough fields in " + `line`)
+            raise ValueError("Not enough fields in %r" % line)
         function, type, arg, refcount, comment = parts
         if refcount == "null":
             refcount = None