Drop Sphinx compatibility to < 6

[agjohnson]

Sphinx 5.0 Sphinx 6 is dropping jQuery support, which we have historically piggybacked off of. Removing jQuery support here (and in RTD) is totally valid and a good goal as well, we're not doing anything with jQuery that we couldn't do by hand. But to start, we should squeak out a release that pins our Sphinx < 5 so we don't risk a theme version that doesn't have jQuery underneath.

We can discuss a few options for removing jQuery, including porting code and async loading of jQuery when it doesn't exist through webpack (we use this feature in our new site templates).

Related #1253

[astrojuanlu]

But to start, we should squeak out a release that pins our Sphinx < 5 so we don't risk a theme version that doesn't have jQuery underneath.

Indeed!

[felixoi]

As Sphinx 5.0 is released, do I read this correct that currently rtd_theme is not compatible with it?

EDIT: As per changelog the removal happens ongoing from 6.0, correct?

jQuery and underscore.js will no longer be automatically injected into themes from Sphinx 6.0.

[stsewd]

EDIT: As per changelog the removal happens ongoing from 6.0, correct?

yes, 6.0 will drop jquery, not 5.x