Use new Vega loading mechanism from synphot 1.7.0

[pllim]

Description

This pull request is to fix #222 using functionality added in spacetelescope/synphot_refactor#435 but without breaking existing API.

cc @hamogu

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.07%. Comparing base (2dbdc60) to head (2601dd7).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
stsynphot/spectrum.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage   84.51%   84.07%   -0.44%     
==========================================
  Files          13       13              
  Lines        1621     1633      +12     
==========================================
+ Hits         1370     1373       +3     
- Misses        251      260       +9     

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