Investigate errors caused by Dask 2024.12 and later

[trexfeathers]

🐛 Bug Report

This will be temporarily solved by #6255, but we will be unable to unpin again until this problem is solved.

Test failures:

• test_pickle.py: TypeError: a class that defines __slots__ without defining __getstate__ cannot be pickled
  • All file formats (including GRIB - see [CI Bot] environment lockfiles auto-update iris-grib#550)
  • Protocols 0 and 1 (but not 2)
• test__interpret_field.py::Test__interpret_fields__land_packed_fields::test_landsea_unpacking_uses_dask: TypeError: 'Task' object is not subscriptable

How To Reproduce

Steps to reproduce the behaviour:

1. Remove any dask-core maximum pins in iris/requirements/py312.yml
2. conda create -n tmp-dask --file iris/requirements/py312.yml
3. Run the above tests in these environments - the failures appear
4. conda install "dask<2024.12"
5. Re-run the tests - the failures do not appear

Expected behaviour

All tests should pass

Environment

• OS & Version: [e.g., Ubuntu 20.04 LTS]
• Iris Version: [e.g., From the command line run python -c "import iris; print(iris.__version__)"]

See above for environments. Experienced with the Iris main branch.