Doc tests are failing in CI on the rustc beta channel: example

To Reproduce
Submit any PR that triggers the CI workflow in Github actions.

Expected be 5A4E havior
Doc tests pass

Notes

This is probably caused by skeptic, which appears to be unmaintained. In order to get doc tests working, it's probably necessary to either:

1. Fix in skeptic and hope the maintainer is available to publish a release
2. Switch to [builtin documentation tests. I'm not sure what special features of skeptic are used, or how much work the transition would take.