skymap_from_samples failing due to too many files open

As mentioned in this Sentry issue, gwcelery.tasks.skymaps.skymap_from_samples jobs on gracedb-playground are failing due to too many files being open (in this case we wanted to create a temporary file).

This issue is currently blocking the deployment of gwcelery 2.1.7 since we need to verify that the release is not responsible for this error.