David, I don't think it's that - I think it's the subprocess comms. This works:

    def _do_test(self, program, exc_text):
        with open(self.testfn, 'w') as testmod:
            testmod.writelines(program.format(
                exception_action=self.exception_action))
        p = subprocess.Popen([sys.executable, 'testmod.py'],
                              stderr=subprocess.PIPE)
        streams = p.communicate()
        v1 = streams[1].decode('utf-8') # this shouldn't be hardcoded!
        v2 = exc_text.format(exception_action=self.exception_action)
        self.assertEqual(v1, v2)

But I don't think the 'utf-8' encoding should be hardcoded. Not sure what to use - sys.getfilesystemencoding()? locale.getpreferredencoding()? 

Decisions, decisions :-(