Added missing cupy test mark and fixed cupy threshold

[m-albert]

This PR fixes #327 by

• adding the cupy mark to the ignored test
• passing sigma in threshold_local (gaussian mode) as a numpy array instead of as an array of the same type as the input image.

[m-albert]

Looks like the test shows up and succeeds: https://gpuci.gpuopenanalytics.com/job/dask/job/dask-image/job/prb/job/dask-image-prb/131/CUDA_VER=11.5,LINUX_VER=ubuntu18.04,PYTHON_VER=3.10/testReport/

[m-albert]

On main, sigma which is later passed to gaussian_filter is created as a cupy backed dask array when the image is a cupy backed dask array.

sigma = (da.asarray(block_size, like=image._meta) - 1) / 6.0

This creates the error

???
E TypeError: Implicit conversion to a NumPy array is not allowed. Please use .get() to construct a NumPy array explicitly.
cupy/_core/core.pyx:1480: TypeError

I think the fix proposed here makes sense but I might be missing something:

sigma = (np.array(block_size).astype(float) - 1) / 6.0

[GenevieveBuckley]

Nice catch!