DM-21861 adds basic Gen3 butler support for obs_sdss (sufficient for ingest), but we'll need to expand that to get to the point where we can run single-frame processing via Gen3 on it.  That will include, at least:

• implementing writeCuratedCalibrations in the instrument class;
• specializing (and modifying, as necessary) the instrument-specific hooks for the gen2to3 tool, at least to the point where we can write scripts like the ones in ci_hsc_gen3/bin.src to make a fully-usable Gen3 repo.

SDSS is quite different from most other obs packages because we rely on SDSS Photo pipeline outputs for several different calibrations rather than our own.  That means this ticket will need to involve cooking up ways to ingest those files into Gen3 repos and writing PipelineTask versions of any SDSS-specific CmdLineTasks we use.