RTD build.image Key is Deprecated

[derks]

From RTD:

The build.image config key on .readthedocs.yaml has been deprecated, and will be removed on October 16, 2023. We are sending weekly notifications about this issue to all impacted users, as well as temporary build failures (brownouts) as the date approaches for those who haven't migrated their projects.

Please use build.os on your configuration file to ensure that they continue building successfully and to stop receiving these notifications.

[derks]

Not sure what this is about... we are using build.os already:

• https://github.com/datafolklabs/cement/blob/main/readthedocs.yml#L3