Remove _scripts, use new PyPI module instead

[matthew-brett]

I've refactored the notebook post-processing into the jljb module.

[jarrodmillman]

Sorry, I missed something. Are we still testing on multiple versions of Python?

[matthew-brett]

You mean - why did I delete test.yml? Jupyter Book doesn't support the doctesting that a bare Sphinx build does. In a way, that's a boon, because there was some confusing markup that the user sees, in order to satisfy doctest. But in any case, we can't test in the same way.

I did put in some test machinery though, that you can see described in CONTRIBUTING.md - in the form of cells marked remove-cell with asserts in them. These don't appear in the built output or the JupyterLite notebooks, but do get run as part of the book build process.