Experiment with Cover Up to increase Python test coverage

https://github.com/plasma-umass/coverup

Related:

* https://github.com/open-contracting/kingfisher-process/issues/328
* https://github.com/open-contracting/data-registry/issues/128
* https://github.com/open-contracting/pelican-backend/issues/20
* https://github.com/open-contracting/pelican-frontend/issues/12
* https://github.com/open-contracting/extension-explorer/issues/41