Bump notebook from 6.0.2 to 6.1.5

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 6.0.2 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] <support@github.com>

Author: dependabot[bot]

build_requirements.txt

@@ -1,7 +1,7 @@
 setuptools-scm==3.5.0
 mkdocs==1.1.2
 mkdocs-jupyter==0.13.0
-notebook==6.0.2
+notebook==6.1.5
 pymdown-extensions==6.3
 nbconvert==5.6.1
 git+https://github.com/tomchristie/mkautodoc.git#egg=mkautodoc