`pip install cuda-core[cu13]` does not exist

[leofang]

See the CI log:

WARNING: cuda-core 0.3.1 does not provide the extra 'cu13'
WARNING: cuda-core 0.3.1 does not provide the extra 'test-cu13'

pip's behavior is to ignore nonexistent extra dependencies and move on, which is not what we want. I guess we'll have to cut a v0.3.2 release for cuda.core.