Fix compatibility with future glue-core changes

[astrofrog]

Similar to glue-viz/glue-jupyter#380

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (a4476e0) 96.89% compared to head (a83efdc) 96.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   96.89%   96.90%   +0.01%     
==========================================
  Files          18       18              
  Lines        1415     1421       +6     
==========================================
+ Hits         1371     1377       +6     
  Misses         44       44              

Files Changed	Coverage Δ
glue_astronomy/io/spectral_cube/spectral_cube.py	84.09% <100.00%> (+1.16%)	⬆️
...onomy/io/spectral_cube/tests/test_spectral_cube.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.