Require CuPy 9.0.0+

[jakirkham]

At this point CuPy 9.0.0 has what we need as discussed in PR ( #325 )

Perhaps we should update requirements and importorskip lines to match

[m-albert]

Thoughts:

1. importorskip: Currently the importorskip lines indicate the min version required for the specific functionality they're associated to. Would it be preferable to have them pinned to 9.0.0 generally?

2. requirements: In my current understanding the only place we're indicating information about CuPy versions is in the release notes in HISTORY.rst since cupy is not a dependency.

[jakirkham]

On 2, we might want to make add cupy as an optional dependency with a version constraint. This is partially motivated by simply documenting what we intend to support (though it may also act as an installation convenience)

[m-albert]

Great idea. To have it linked here, @GenevieveBuckley is incorporating cupy as an optional dependency into the pyproject.toml #306

[GenevieveBuckley]

Closed by https://github.com/dask/dask-image/pull/306/files
See:

dask-image/pyproject.toml

Line 48 in 8393093

"cupy >=9.0.0",