CI: tests vs. cupy

[crusaderky]

#53 add tests against all backend libraries known to array-api-compat.
cupy however has been excluded from running in CI, because free github actions don't offer GPU hosts.

CC @rgommers

[rgommers]

Note that I'm working on this in scipy/scipy#21740. It works pretty nicely overall, except that on some CUDA errors the Cirrus Runners infra disconnects and eats the CI log of the failing step rather than showing the error. That is the second of two important errors, the first one was fixed by the Cirrus folks already. I'm hoping they come through on the second one, and then it should be good to go (if not, it's still mergeable, but not nearly as nice).