Fix dask axyncio.exceptions.CancelledError

[Lukas113]

When using dask (e.g. through pytest), sometimes a asyncio.exceptions.CancelledError occurs. According to dask-issue 3275 I think it is because the LocalCluster is not closed properly in Karabo.

I'm not sure what the implication of this issue is, meaning if it can cause any harm to some functionalities or if it just results in an ugly stderr output.

[Lukas113]

Maybe I can elaborate a little bit more on this issue. This is something, that occurs on a regular basis on the CI scheduled tests, which can be seen at the very end of the according CI-job. If I understood @kenfus correctly, this is just ugly stderr output and shouldn't have any negative impact, right? Even if that's the case, it still feels that we're doing something wrong and should tackle this issue.