Make essm compatible with Python 3.10

[schymans]

setup.py test and pytest-runner are deprecated for security reasons (https://pypi.org/project/pytest-runner/). Need to update unit testing following current best practice, e.g. https://docs.pytest.org/en/stable/goodpractices.html.

[schymans]

This issue actually prevents me from creating a conda build of essm: conda-forge/staged-recipes#12425
The error is caused by the inability to install pytest-runner.
@jirikuncar could you help updating the unit testing of essm, or tell me where to start? Thanks!

[schymans]

Pasting a PM from @jirikuncar, for the records:
'First, I think we should look at which Python version we want to support and migrate setup.py to pyproject.toml'

The goal should be to make essm compatible with Python 3.10.