Remove numba dependency

[gmarkall]

EDIT: This now removes the numba dependency, which comes transitively from cuDF.

Original description:

numba-cuda is the NVIDIA-maintained CUDA target for Numba, which depends on the numba package.

This PR replaces the numba dependency with numba-cuda. However, similar to @bdice's comment in #648, I don't know if we strictly need this here any more due to the cudf dependency (from rapidsai/cudf#16474) ensuring numba-cuda is present.