Fix the Zenodo RST utility module
This PR fixes the gwpy.utils.sphinx.zenodo
API interface module uses to populate the citing
page on the online docs (e.g. here), which was broken by a recent (apparently undocumented) Zenodo API change (see https://github.com/zenodo/zenodo/issues/2483).
I also took the opportunity to refactor the module to improve the layout, and added a test module.