Prepare for python3.14

[arrjon]

Small refactor so multiprocessing callables are module-scope and pickle-friendly. Problem mentioned in #665

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Codecov Report

❌ Patch coverage is 90.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.29%. Comparing base (5f2b297) to head (65bfd8e).

Files with missing lines	Patch %	Lines
pyabc/inference_util/inference_util.py	94.11%	1 Missing and 1 partial ⚠️
pyabc/sampler/util.py	60.00%	1 Missing and 1 partial ⚠️
pyabc/sampler/redis_eps/cli.py	50.00%	1 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #666      +/-   ##
===========================================
+ Coverage    79.18%   79.29%   +0.10%     
===========================================
  Files          141      141              
  Lines         8327     8337      +10     
  Branches      1110     1111       +1     
===========================================
+ Hits          6594     6611      +17     
+ Misses        1363     1357       -6     
+ Partials       370      369       -1     

Flag	Coverage Δ
base	59.44% <90.00%> (+0.13%)	⬆️
external-R	36.82% <82.00%> (+<0.01%)	⬆️
external-other-simulators	31.46% <42.00%> (+0.03%)	⬆️
migrate	25.44% <28.00%> (+0.08%)	⬆️
petab	35.51% <56.00%> (+0.01%)	⬆️
visualization	52.79% <44.00%> (+0.16%)	⬆️

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