re-add specutils to downstream #2283

Job	Run time
asdf / Load tox environments	5s
astropy / Load tox environments	4s
stsci / Load tox environments	5s
third-party / Load tox environments	5s
asdf / asdf-wcs-schemas (ubuntu-latest)	1m 24s
asdf / asdf-coordinates-schemas (ubuntu-latest)	2m 26s
astropy / asdf-astropy (ubuntu-latest)	3m 1s
astropy / specutils (ubuntu-latest)	2m 10s
stsci / gwcs (ubuntu-latest)	1m 29s
stsci / jwst (ubuntu-latest)	20m 18s
stsci / stdatamodels (ubuntu-latest)	2m 7s
stsci / stpipe (ubuntu-latest)	44s
third-party / weldx (ubuntu-latest)	2m 53s
stsci / roman_datamodels (ubuntu-latest)	2m 11s
third-party / sunpy (ubuntu-latest)	3m 14s
third-party / dkist (ubuntu-latest)	4m 12s
	46m 28s

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