Switch default backend to ProcessPoolExecutor even when loky is installed.

[tobiasraabe]

No description provided.

[codecov]

Codecov Report

Merging #59 (5678002) into main (87d0c6f) will increase coverage by 0.22%.
The diff coverage is 100.00%.

❗ Current head 5678002 differs from pull request most recent head 615f160. Consider uploading reports for the commit 615f160 to get more accurate results

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   94.44%   94.67%   +0.22%     
==========================================
  Files          10       10              
  Lines         414      413       -1     
==========================================
  Hits          391      391              
+ Misses         23       22       -1     

Flag	Coverage Δ
end_to_end	87.89% <100.00%> (+0.21%)	⬆️
integration	50.12% <100.00%> (+0.12%)	⬆️
unit	46.48% <100.00%> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
src/pytask_parallel/backends.py	72.41% <100.00%> (+2.41%)	⬆️