dont use loky backend in tests

[knaaptime]

resolves #257

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.58%. Comparing base (3d0ab2f) to head (2e91fbe).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #256      +/-   ##
==========================================
+ Coverage   81.79%   82.58%   +0.78%     
==========================================
  Files          66       66              
  Lines        2302     2302              
==========================================
+ Hits         1883     1901      +18     
+ Misses        419      401      -18     

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