Add scikit-image dependency to setup.py

[jni]

Installing napari and ome-zarr results in a broken plugin, because napari does not depend on scikit-image.

[joshmoore]

Ta. I'll roll this into the impending release.

[sbesson]

Thanks @jni I also bumped into this issue while consuming ome-zarr. I will test things work with the latest release without having to redeclare scikit-image in downstream modules.