Use pytest to run the tests

It has nicer test output and isn't deprecated. We still use nose.tools for asserts for the moment.

We merge the doctest as the module level fixtures don't work with pytest. We lose the linking to the tested functions but that's not that much of a loss.

We mark two rpm related tests as xfail for the moment as they fail for unrelated (but yet unknown reasons) as a patch number goes missing in the spec file.