spack + python virtualenv is brittle

I have already hit some issues with this:

• when the python required in the spack env is not the one in the virtual env, weird things happen
• when spack is installed or the shell initialised and a virtual env is active, spack then does not work and fails with the spack: command not found error

On the other hand, it takes much less time to install carputils via pip than via spack install py-carputils, which has been crucial when making attempts.

To simplify the setup, we can remove the python external package/root spec in the environment definition and add instead py-carputils, and remove the python venv setup. Now that things more or less work this can be done probably without too much pain.