MNT: Compatibility with astropy>=5

[pllim]

Description

This pull request is to address compatibility problem with astropy>=5.0, specifically astropy/astropy#11930 .

[codecov]

Codecov Report

Merging #321 (4241c1f) into master (760225e) will decrease coverage by 0.08%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   94.40%   94.31%   -0.09%     
==========================================
  Files          14       14              
  Lines        2000     2005       +5     
==========================================
+ Hits         1888     1891       +3     
- Misses        112      114       +2     

Impacted Files	Coverage Δ
synphot/models.py	84.81% <71.42%> (-0.37%)	⬇️
synphot/compat.py	77.77% <100.00%> (+1.30%)	⬆️

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