Fix deprecation warnings with recent versions of astropy

[astrofrog]

Just a small change to avoid deprecation warnings with recent versions of astropy

[codecov]

Codecov Report

Merging #2212 (483c9f1) into master (ad14432) will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2212      +/-   ##
==========================================
- Coverage   88.03%   88.00%   -0.03%     
==========================================
  Files         247      247              
  Lines       22968    22968              
==========================================
- Hits        20219    20214       -5     
- Misses       2749     2754       +5     

Impacted Files	Coverage Δ
glue/plugins/coordinate_helpers/deprecated.py	44.44% <0.00%> (ø)
glue/plugins/coordinate_helpers/link_helpers.py	100.00% <100.00%> (ø)
glue/viewers/common/state.py	84.21% <0.00%> (-10.53%)	⬇️
glue/conftest.py	59.09% <0.00%> (-3.04%)	⬇️
glue/core/layer_artist.py	80.00% <0.00%> (-0.58%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ad14432...483c9f1. Read the comment docs.