[DISCUSSION] Consider making cupy>=14 a dependency

[ncclementi]

xref #83 (comment)

Starting cupy 14 wheels, have the option on including a ctk, by installing like cupy-cuda12x[ctk]" or "cupy-cuda13x[ctk]" . This makes cupy installed via pip to not depend on a system cuda toolkit., which happens in several systems.

When we install via conda this is not an issue, as conda includes it already.

cupy-xarray, can't work without cupy, at the moment in main cupy is not a dependency, I wonder if the situation described above, makes it easier to implement a reliable path to workable dependencies.

cc: @jacobtomlinson @weiji14

[ncclementi]

@leofang Maybe you have a suggestion, on how to handle this, for a bit more context the dependency was removed because the different builds, i.e cupy, cupy-cuda12x and cupy-cuda13x.

How do other libraries handle this? See extended discussion in #66 (comment)