Just uploaded https://bugs.python.org/file39083/fa4c6160c518.diff. Pretty sure I've addressed all of @berker.peksag's review comments: thanks for that! 

I also fixed a number of subtle bugs in the tests. Pro tip: when asserting that something raises TypeError, inspect the exception message. There are many many ways that Python code can raise TypeError *other than* the condition you thought you were testing for. ;-)

I'm happy with this patch now unless anyone else spots problems.

@durin42: have you been trying this patch with your Mercurial-on-Python-3.5 patches? This would be a good time to re-update your difflib.py.