[Python] Fix unordered SolutionArray append

[ischoegl]

If applicable, fill in the issue number this pull request is fixing

Closes #1838

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.41%. Comparing base (2e0d8ac) to head (6ea5110).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1840   +/-   ##
=======================================
  Coverage   74.41%   74.41%           
=======================================
  Files         382      382           
  Lines       53411    53411           
  Branches     9026     9026           
=======================================
  Hits        39747    39747           
  Misses      10617    10617           
  Partials     3047     3047           

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[speth]

Thanks, @ischoegl. This looks good to me.