FIX: Update PET metadata dependencies

[effigies]

Fixes #1897.

[effigies]

Well, this didn't work. For some reason it's causing failures in non-PET examples. @rwblair I modeled this off of the coordsystem_nirs.json, but it seems that I'm missing something about how the validator works.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.57%. Comparing base (7773eda) to head (1008250).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1906   +/-   ##
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  Coverage   83.57%   83.57%           
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  Files          92       92           
  Lines        3890     3890           
  Branches     1271     1271           
=======================================
  Hits         3251     3251           
  Misses        541      541           
  Partials       98       98           

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