This updates some remaining bits of code in the diff viewer to work with

both Python 2 and 3. This involves:


Switching to alternative methods for chunk building (working around

   behavioral changes in map and partial)


Removing usage of StopIteration (which no longer behaves the same

   way and is deprecated)


Fixing a comparison between a string and an integer in chunk

   generation.


Fixing the building of strings and other data in tests to get the

   correct types and to work with Python 3's formatting restrictions.

Unit tests pass on Python 2.7 and 3.7 (with other in-progress changes).