Softening pyculib_sorting dependency

Am curious whether it would be possible to soften the `pyculib_sorting` dependency? IIUC the code here is pure Python and uses CFFI to interact with CUDA libraries' C API. This makes it pretty easy to package and distribute. Though `pyculib_sorting` seems to make use of CUDA code, which is a bit harder to build, test, and distribute. Softening the `pyculib_sorting` dependency should make this pretty easy to distribute.

Start by locating the dependency declarations and packaging configuration for pyculib_sorting. Review how the Python and CFFI bindings interact with the CUDA libraries and determine whether the dependency can be made optional. Done means the package can be distributed without a hard pyculib_sorting dependency while preserving the existing functionality.

Written by the indexing model from the issue text.