This bumps our version of Sphinx to 5.3, and our version of

beanbag-docutils to 2.1.1.
Sphinx 5.3 is not the latest version of Sphinx (it's currently at

6.1.3), but it is the latest to support building with Python 3.7.
beanbag-docutils 2.1.1 has fixed support for multi-DPI images, which are

now enabled for our docs (replacing the old Retina image extension).

Successfully installed the new dependencies and built the docs.
Verified that all images with @2x variations now render using

<img srcset> with both 1x and 2x versions.