rename Febus named tuple to private

[d-chambers]

Description

The named tuple in the Febus utils module is messing with the documentation build since the doc build system can't get its source. Adding an underscore in the name solves the issue.

Checklist

I have (if applicable):

• [ ] referenced the GitHub issue this PR closes.
• [ ] documented the new feature with docstrings or appropriate doc page.
• [ ] included a test. See testing guidelines.
• [ ] your name has been added to the contributors page (docs/contributors.md).
• [ ] added the "ready_for_review" tag once the PR is ready to be reviewed.