Fix: synphot keyword argument deprecation

[mkelley]

• Only use flux_unit and wave_unit with synphot's read_*_spec when requested.
• Respect the cache parameter on SpectralSource.from_file

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.83%. Comparing base (59a8a49) to head (6515bd2).

Files with missing lines	Patch %	Lines
sbpy/spectroscopy/sources.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
==========================================
+ Coverage   81.81%   81.83%   +0.01%     
==========================================
  Files          53       53              
  Lines        4267     4271       +4     
==========================================
+ Hits         3491     3495       +4     
  Misses        776      776              

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[mkelley]

Ready for review. That uncovered line is covered by the remote tests. Maybe we should enable them on PRs again. (although a few of them are currently failing)