forking for new processes is discouraged and might lead to deadlocks in the child.

[maurerle]

See python/cpython#84559 and of course
https://pythonspeed.com/articles/python-multiprocessing/

This does only fix the warning which comes up starting from python 3.12 - this config should be used in user code in the beginning somewhere.
We could also set this manually in mango generally, though this leaves the user less choice.

When using the spawn method instead of fork, all process memory has to be pickled first, leaving extra steps when using the serializer methods currently..

I might improve this in this PR as well

[maurerle]

This removes multiprocessing warning but breaks pickling of enum in other tests, when all tests are executed in one run, so I'll close this for now..