Embrace SPEC 0 Minimum Supported Version https://scientific-python.org/specs/spec-0000/

Akin to Numpy and Scipy, I propose to only support recent versions of python, numpy, and scipy (released within the last 3 years) in development branches. See https://scientific-python.org/specs/spec-0000/ for details on the joint effort of the scientific community to standardize this. We would continue to support older versions of NIFTy which work with older versions of python, numpy, and scipy but the development version need only to work with SPEC 0 compliant versions. Specifically, this would drop python3.8 support. @all