Upgrade to jupyer-book >=0.15

[mwcraig]

Right now the jupyter-book version is (or will shortly be) pinned to 0.14.x 0.13.x
because of a an upgrade of sphinx-book-theme in jupyter-book 0.15 that eliminates the extra_navbar. I think this can be fixed by using primary-sidebar-end-sections instead of extra_navbar.

Turns out 0.14.0 depends on version 0.4..0rc1 of the book theme, and 0.4.0 was never released.

[mwcraig]

I think a different fix than the one in the comment above is needed, or I need to better understand how that works. In any event, the underlying issue still exists in jupyter-book v1 so we need either a different fix altogether for editions or to make the workaround above work.