Code review comment for lp:~cjwatson/storm/py3-scoping

Formally Storm still supports Python 2.6 (that is, we've specifically dropped support for earlier versions but not yet for 2.6). The form of assertRaises that's usable as a context manager to allow performing additional checks on the exception raised was only introduced in 2.7. Although I don't particularly care about 2.6, it felt less invasive to fix this up within the existing version constraints.