Add spack package file to the repository (#1466)
Add spack package file to the repository (#1466)

Replicate the spack `package.py` from the main spack repository into the arbor repo.

Some small updates were made:
- Correctly provide a url for the GitHub tagged release
- Specify minimum pybind11 version 2.6

Fixes #1465 
halfflat/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,halfflat/arbor,eth-cscs/nestmc-proto,eth-cscs/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto