MNT: Compatibility with specutils 2.0

[pllim]

Description

This pull request is to fix #411

[github-actions]

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• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the STScI coding guidelines?
• Are tests added/updated as required? If so, do they follow the STScI testing guidelines?
• Are docs added/updated as required?
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[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.14%. Comparing base (f0b162e) to head (5500f9e).
⚠️ Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
synphot/compat_specutils.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
- Coverage   97.18%   97.14%   -0.04%     
==========================================
  Files          17       18       +1     
  Lines        2026     2035       +9     
==========================================
+ Hits         1969     1977       +8     
- Misses         57       58       +1     

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