Prepare for jupyter_core v6

[ptsavol]

Running the tests gives the following warning

        DeprecationWarning: Jupyter is migrating its paths to use standard platformdirs
          given by the platformdirs library.  To remove this warning and
          see the appropriate new directories, set the environment variable
          `JUPYTER_PLATFORM_DIRS=1` and then run `jupyter --paths`.

          The use of platformdirs will be the default in `jupyter_core` v6
          from jupyter_core.paths import jupyter_data_dir, jupyter_runtime_dir, secure_write

This means that kernel search paths will change. Eg. on Windows, new Python kernels will be created into %appdata%/Local/Jupyter/kernels instead of the current %appdata%/Roaming/jupyter/kernels. If we don't do anything, Spine Toolbox won't find the current Jupyter kernels anymore when a new jupyter_client drops that requires jupyter_core v6.

See linked two PR's:

• Set env variable JUPYTER_PLATFORM_DIRS #3123
• Add legacy jupyter-core kernel search path into kernel search paths spine-engine#173

Note: The PR's above are ready to go in case the platformdirs change is actually part of jupyter-core v6. See Issue 309 in jupyter-core repo https://github.com/jupyter/jupyter_core/issues/ for additional info.