Backport PR #16709 on branch v6.1.x (Reduce duplication of CompoundModel._initialize_slices) - #16762

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I don't understand why Windows is failing on FITS stuff in this backport.

Also since 6.1.2 is released but @astrofrog plans to do a post-release, I am also not sure on the milestone. 🤷‍♀️

Maybe the Windows failure is not related.

Since Tom R is in the midst of release, I will let him merge.