Bump notebook from 7.5.5 to 7.5.6

Bumps [notebook](https://github.com/jupyter/notebook) from 7.5.5 to 7.5.6.
- [Release notes](https://github.com/jupyter/notebook/releases)
- [Changelog](https://github.com/jupyter/notebook/blob/@jupyter-notebook/tree@7.5.6/CHANGELOG.md)
- [Commits](https://github.com/jupyter/notebook/compare/@jupyter-notebook/tree@7.5.5...@jupyter-notebook/tree@7.5.6)

---
updated-dependencies:
- dependency-name: notebook
  dependency-version: 7.5.6
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]