Merge lp://staging/~sseman/juju-ci-tools/model-change-watcher-py3-2 into lp://staging/juju-ci-tools

I'll wait until the pre-req is landed. However, 1 comment below.

Thank you. I think the Makefile changes are incomplete.

I don't know if it works with all OSes you mentioned but I do see your point.

Instead of breaking "make test", I can add a different rule for Python 3 unit tests "test_python3" and only run tests under "tests_py3" directory.

Instead of using the find, grep, xargs commands for lint test, I will exclude the files explicitly and add a different rule for Python 3.
 py3="assess_model_change_watcher.py test_assess_model_change_watcher.py"
 lint:
    flake8 $$(find -name '*.py') --builtins xrange,basestring --exclude $(py3)
 lint_python3:
    flake8 $(py3)

Does the above seem a reasonable solution?

Thank you Seman.