Remove `PYTHON_VERSION_JUPYTER` and default Jupyter installs to the primary or alternate Python version

[Ian Pittwood]

Closes #613

Relates to #590

Having PYTHON_VERSION_JUPYTER as a separate version can be confusing to end users since that variable does not specifically install a different version, it only references a that version for the path used when creating the Jupyter sandbox virtual environment. We should create a convention for Jupyter to always use the primary or alternate Python and update all projects accordingly.

          This confused me, as I thought that it was a third installation of python. Why are we explicitly setting this? Why isn't it just always presumed to be installed at PYTHON_VERSION?

Originally posted by @msarahan in #564 (comment)

[Benjamin R. J. Schwedler]

I really like this approach. The more variables we can get rid of, the better!