re-add specutils to downstream #2266

Job	Run time
asdf / Load tox environments	6s
astropy / Load tox environments	7s
stsci / Load tox environments	4s
third-party / Load tox environments	10s
asdf / asdf-wcs-schemas (ubuntu-latest)	1m 52s
asdf / asdf-coordinates-schemas (ubuntu-latest)	2m 26s
stsci / gwcs (ubuntu-latest)	1m 21s
stsci / jwst (ubuntu-latest)	18m 28s
stsci / stdatamodels (ubuntu-latest)	4m 7s
stsci / stpipe (ubuntu-latest)	58s
stsci / roman_datamodels (ubuntu-latest)	1m 29s
astropy / asdf-astropy (ubuntu-latest)	3m 3s
astropy / specutils (ubuntu-latest)	2m 0s
third-party / weldx (ubuntu-latest)	2m 52s
third-party / sunpy (ubuntu-latest)	2m 30s
third-party / dkist (ubuntu-latest)	5m 58s
	47m 31s

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