re-add specutils to downstream #2301

Job	Run time
astropy / Load tox environments	7s
asdf / Load tox environments	8s
stsci / Load tox environments	9s
third-party / Load tox environments	10s
asdf / asdf-wcs-schemas (ubuntu-latest)	1m 29s
asdf / asdf-coordinates-schemas (ubuntu-latest)	3m 15s
astropy / asdf-astropy (ubuntu-latest)	4m 6s
astropy / specutils (ubuntu-latest)	1m 54s
stsci / gwcs (ubuntu-latest)	1m 8s
stsci / jwst (ubuntu-latest)	22m 33s
third-party / weldx (ubuntu-latest)	3m 0s
stsci / stdatamodels (ubuntu-latest)	3m 17s
third-party / sunpy (ubuntu-latest)	2m 53s
stsci / stpipe (ubuntu-latest)	50s
third-party / dkist (ubuntu-latest)	4m 56s
stsci / roman_datamodels (ubuntu-latest)	2m 12s
	52m 7s

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