Bump notebook from 5.7.4 to 6.1.5 in /infra/pyspark-jupyter

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 5.7.4 to 6.1.5.
- [Release notes](https://github.com/jupyter/jupyterhub/releases)
- [Changelog](https://github.com/jupyterhub/jupyterhub/blob/master/CHECKLIST-Release.md)
- [Commits](https://github.com/jupyter/jupyterhub/commits)

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

Author: dependabot[bot]

infra/pyspark-jupyter/requirements.txt

@@ -18,7 +18,7 @@ MarkupSafe==1.1.0
 mistune==0.8.4
 nbconvert==5.4.0
 nbformat==4.4.0
-notebook==5.7.4
+notebook==6.1.5
 numpy==1.16.1
 pandas==0.24.1
 pandocfilters==1.4.2