fix python version incompatibilities/requirements
The minimum required python version for the project is currently 3.8!
fluiddyn/clusters/slurm.py
'subprocess.run(text)' requires 3.7
fluiddyn/util/compat.py
'functools.cached_property' member requires 3.8
Either fix the incompatibility or update the documentation to reflect this requirement. Add a CI test to enforce the documented minimum required python version.