Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/base64.py b/Lib/base64.py
index 54ee623..f90b91d 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -348,7 +348,7 @@
     s0 = "Aladdin:open sesame"
     s1 = encodestring(s0)
     s2 = decodestring(s1)
-    print s0, `s1`, s2
+    print s0, repr(s1), s2
 
 
 if __name__ == '__main__':