Investigate discrepancy of running unit and cli tests between py311 and py312

[xjules]

It is apparent that Python 3.12 is much slower than Python 3.11 when it comes running the all tests. We should find out what's the main source of this slowness.
Unit tests: Py311 is 3-4 minutes faster
Cli tests: Py311 is 4-5 minutes faster
Check here for more info https://github.com/equinor/ert/actions/runs/11971902477/job/33377669575

[JHolba]

Async generators seem to be slower

https://en.lewoniewski.info/2023/python-3-11-vs-python-3-12-performance-testing/