Fix cupy pytest errors

[GenevieveBuckley]

Followup to PR #366 (I forgot to make equivalent changes for the cupy tests).

[GenevieveBuckley]

This PR is necessary, but not sufficient alone to fix #369, I think there might be additional problems causing the GPU CI failures.

[GenevieveBuckley]

Status:

• GPU CI python 3.9 - fatal python error in the rank filter cupy test tests/test_dask_image/test_ndfilters/test_cupy_ndfilters.py::test_cupy_order[rank_filter-5-3] Fatal Python error: Aborted
• GPU CI python 3.10 - the whole pytest suite seems to pass, but somehow the jenkins GPU CI job is still failed? I can't figure out which part causes the failure (but maybe I'm just too tired to be looking at this right now)

I don't have a GPU handy to debug this right now, I might have to leave it.

[GenevieveBuckley]

Hooray, the GPU CI is fixed!