Add mechanism for comparison of setup.py and its equivalent setup.cfg

[amy-lei]

Summary of changes

Closes #1699

Add the sphinx-inline-tabs extension to allow tabbed containers for comparing setup.py and setup.cfg. The tabs works even when JS is disabled. This PR also replaces instances of adjacent code-blocks with the new extension.

However, I wasn't sure if there was an issue with the default styling. It currently looks like this:

The only thing that stood out and possibly needed overriding was the accent color of the tabs.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[jaraco]

This is beautiful. I honestly did not expect this change to be possible, but the solution you found and implemented is simple and elegant. Thoroughly impressed and grateful.

[webknjaz]

Merging this PR has broken the RTD setup 3 weeks ago. Since then, all the builds are failing: #2614.
I think we should enable the PR RTD builds toggle (#2469) to notice such problems early.