- Add copyright headers to files that did not already have them
- Update the year to the last year the file was modified in files with
existing headers.
During Python 3 migration, the izip call inside diff_interrupt_files has
been replaced with a zip call (zip returning an iterator in Python 3).
Import zip from builtins to ensure that it also produces an iterator in
Python 2.
Also fix the associated unit test by importing the correct function.
