Historically, beanbag-docutils has supported a wide range of Sphinx

versions. This was necessary for our own doc viewing requirements on

reviewboard.org, and generally because maintenance has been easy, up

until recently. That's changing with newer Python versions and with

dependencies that older versions of Sphinx depended on, and as such it's

time to move on.
We now support the latest Sphinx 5.x through 7.3.x. This covers the

range of versions our own codebases currently use. We're also dropping

Python versions prior to 3.8, and adding 3.13.
Packaging has also been updated to be defined in pyproject.toml,

giving us a modern base for the 3.0 release.

All unit tests pass in tox.