The primary reason for this is to support other backends that don't use numba.

It is also valid to use SuiteSparse:GraphBLAS with limited functionality without numba. Other advantages of not requiring numba:

• able to support newer Python versions faster (see Support Python 3.11 #324)
• able to support PyPy (probably)

I think it's still TBD how we'll support multiple backends and how packages and dependencies will be installed.

Acceptance for closing this issue as complete will be to run tests in CI w/o numba installed and to have good error messages when trying to use features that require numba. Numba may still be listed as a required dependency.