Bump nbconvert from 7.8.0 to 7.9.1

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 7.8.0 to 7.9.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Changelog](https://github.com/jupyter/nbconvert/blob/main/CHANGELOG.md)
- [Commits](jupyter/nbconvert@v7.8.0...v7.9.1)

---
updated-dependencies:
- dependency-name: nbconvert
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>