Support ipykernel>=7

[dragon-archer]

ipykernel has officially released version 7 for since Oct. 2025.
How about removing the ipykernel<7 in pyproject.toml?

[ccordoba12]

That's not so simple. We have to run our test suite here and in Spyder to check everything is working as expected.

For major versions that's usually not the case, which requires more time to fix errors and rerun tests again. Unfortunately, we don't have time to do that at the moment, sorry (perhaps in a couple of months).