⬆ UPGRADE: Pydata Sphinx Theme v0.6.0

[choldgraf]

This is a first pass at upgrading to the latest pydata sphinx theme, and removing our custom sidebar collapsing code to use it instead. This should remove a bunch of our CSS + __init__.py code, as well as some testing infrastructure for the sidebar collapsing logic.

Note - this will remove one feature for us, which is navbar_depth = 0. This would have removed all navbar items except for captions (and made them collapsible). The pydata theme does not support collapsible captions so this will remove this functionality. That said:

• I did a github search and couldn't find any configuration that was using navbar_depth = 0
• I also looked through the Jupyter Book docs and don't think we've documented that anywhere

so I think it is OK to remove, but welcome others suggesting otherwise!

closes #323

ToDo

• Make the tests pass
• Make sure that the visual look is the same as it was before (or better)
• Review to make sure the look is correct, and we're not missing anything

[choldgraf]

OK updated the tests and cleaned up some of the code. I think that this is ready to go whenever we'd like to merge.

@DrDrij let me know if you'd like me to hold off merging so that this doesn't mess with some work you're doing.

Also speak now @executablebooks/ebpteam if you really wanted to be able to collapse the sidebar TOC entirely (so that only captions are shown and no links) via navigation_depth = 0 since that is being removed here.

[DrDrij]

@choldgraf These styles add the slide animation to left sidebar.