Two options on portability, I think: either use splitlines() on the decoded result, or add a 'universal_newlines' keyword argument to assert_python_failure and friends that would get passed to Popen.  I'd favor the latter, as other users of script_helpers may find it useful.  On the other hand, that may involve a whole separate discussion, so you could use splitlines here and we could open a new issue for a script_helpers enhancement :)

Also, rather than a direct re, I'd use assertRegex.  That will give a more useful error message if the test fails.