Modified test to run on python 2 and python 3

The Testcase `SignallingTestCase` verified the changes to be an instance of `list`. With Python3 the return value of `values` on a `dict` does not give a list but a view `dict_values` on the dictionary values.

I have modified the test so, that it does not check the type if the changes, but inspects the tuple.
The first element has to be an instance of `self.Todo` the second has to be one of 'insert', 'update' or 'delete'

With this change the tests run on all four configured travis jobs.