Stop installing documentation files to top-level site-packages

Add `format = sdist` to `include` table values to prevent
the documentation files from being installed directly into
site-packages, i.e.:

    /usr/lib/python3.11/site-packages/CHANGELOG.md
    /usr/lib/python3.11/site-packages/LICENSE

Originally reported by Anna Vyalkova on https://bugs.gentoo.org/887569.