Remove all code connected with installing python versions

[mattip]

These days there are better ways of installing python versions on CI systems. Most systems come with versions installed (like manylinux dockers), or provide integrated utilities for installing python (like github action's setup-python).

As an interim step, we could isolate the install_*python functions and scripts and their helpers, along with the relevant tests. Perhaps moving them to a subdirectory. This would eliminate fragile code like the macos sdk detection.