Improve Zenodo fixtures and add caching in GitHub CI

[tcompa]

Checklist before merging

• [x] I added an appropriate entry to CHANGELOG.md

[github-actions[bot]]

Coverage report

The coverage rate went from 90.5% to 90.5% :arrow_up: The branch rate is 84%.

None of the new lines are part of the tested code. Therefore, there is no coverage data about them.

[tcompa]

As shown in 4c394ed and in the caching mechanism in GitHub actions does prevent a second download from within the pytest CI.

This means that the download times are under control, and they are visible in the GitHub CI logs, see for instance:

This does not necessarily make test much faster, but it should greatly reduce fluctuations in their duration due to slow downloads from Zenodo.