Reactivate cache in python CI

We had previously deactivate cache in python CI in order to try figure out why tests were not running and failing (cf https://github.com/refstudio/refstudio/pull/63#discussion_r1213541805). Now we have confirmed the CI is working properly, we can reactivate cache to make the CI workflow faster