Not important right now, but this could be made more readable as:

```
    @pytest.mark.parametrize(('section', 'expected_urls'),
                             [(None, M31_URL_ALL),
                              ('e', M31_URL_E),
                              ('em', M31_URL_E),
                              ('emission', M31_URL_E),
                              ('r', M31_URL_R),
                              ('red', M31_URL_R),
                              ('reddening', M31_URL_R),
                              ('t', M31_URL_T),
                              ('temp', M31_URL_T),
                              ('temperature', M31_URL_T)
                              ])
     ```