Issue 3189: Py3k DeprecationWarning in difflib

When running with the -3 flag, difflib creates DeprecationWarnings. I 
attach a patch fixing them.

There was a note in the code saying "DOES NOT WORK for x in a". However, 
after my changes, tests still pass, so I removed these notices. If someone 
knows what this was all about, let me know.