threads-and-processes.md: AttributeError in exercises with multiprocessing

[wmotion]

Replicating the exercises

1. https://esciencecenter-digital-skills.github.io/parallel-python-workbench/instructor/threads-and-processes.html#exercise-reimplement-calc_pi-to-use-a-queue-to-return-the-result
2. https://esciencecenter-digital-skills.github.io/parallel-python-workbench/instructor/threads-and-processes.html#exercise-adapt-the-original-exercise-to-submit-tasks-to-a-pool

throws the error:

AttributeError: Can't get attribute 'calc_pi' on <module 'main' (built-in)>

while the cell seems to be at work (note the mark [*] below) somehow indefinitely (problem size: 10^6). Interrupting the kernel with Jupyter Lab causes a KeyboardInterrupt, not unexpectedly.

The attribute error is attributed to bad relationships between multiprocessing and iPython in https://stackoverflow.com/questions/41385708/multiprocessing-example-giving-attributeerror/42383397#42383397

Python 3.8.16
Jupyterlab 3.5.3
Conda 25.3.0