use explicit multiprocessing context for creating Pipe in subproc.py

[maxhuettenrauch]

• I have added the correct label(s) to this Pull Request or linked the relevant issue(s)
• I have provided a description of the changes in this Pull Request
• I have added documentation for my changes
• If applicable, I have added tests to cover my changes.
• I have reformatted the code using poe format
• I have checked style and types with poe lint and poe type-check
• (Optional) I ran tests locally with poe test
  (or a subset of them with poe test-reduced) ,and they pass
• (Optional) I have tested that documentation builds correctly with poe doc-build

Not sure why it worked before (or whether it shouldn't have worked), but this seems more consistent...

[MischaPanch]

Is this ready for merge? It's still in draft status

Was a test missing that would have caught this?

[maxhuettenrauch]

Not sure about tests, I'm no multiprocessing expert. All the examples were running, though

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.20%. Comparing base (ca4f74f) to head (a4b6bb4).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1102      +/-   ##
==========================================
- Coverage   88.22%   88.20%   -0.03%     
==========================================
  Files         100      100              
  Lines        8331     8340       +9     
==========================================
+ Hits         7350     7356       +6     
- Misses        981      984       +3     

Flag	Coverage Δ
unittests	88.20% <100.00%> (-0.03%)	⬇️

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