Fixed pystan testing issue.

[MichaelClerx]

Fixes #1739

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (90680a1) to head (5ab77c8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #1740      +/-   ##
===========================================
+ Coverage   99.54%   100.00%   +0.45%     
===========================================
  Files          97        97              
  Lines        9762      9762              
===========================================
+ Hits         9718      9762      +44     
+ Misses         44         0      -44     

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