Fix sage_setup.clean doctest

[mkoeppe]

As reported in https://groups.google.com/g/sage-devel/c/mLtXcaypU3Q/m/jHoVjQ9oAAAJ @kwankyu

This failure was introduced as a side effect of #36591: The pytest discovery of modules disagrees with sage_setup's notion of package directories vs. data directories. We repair it by making sage.tests.books an ordinary package directory (correct because the whole thing is shipped by sagemath-repl).

Fixes #37216

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit c2bb474; changes) is ready! 🎉

[tornaria]

LGTM. https://martinfowler.com/articles/ship-show-ask.html

[mkoeppe]

Thanks!