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Ah, this doesn't quite work as I thought. Configuring an index implies it's always included in resolution, which isn't what we want.

This is now working as intended, with an extremely minor defect: it h5py's nightlies are currently preferred to stable releases. And that's just because it doesn't use dev version numbers (yet). All other nightly packages behave as you'd expect, so even with indexes permanently configured, we only get nightlies when preleases are enabled.