Bump nbconvert from 5.6.1 to 6.5.1

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 5.6.1 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@5.6.1...6.5.1)

---
updated-dependencies:
- dependency-name: nbconvert
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -45,7 +45,7 @@ jupyterlab-widgets==1.0.2; python_version >= '3.6'
 markupsafe==2.0.1; python_version >= '3.6'
 mistune==0.8.4
 nbclient==0.5.10; python_version >= '3.7'
-nbconvert==6.4.0; python_version >= '3.7'
+nbconvert==6.5.1; python_version >= '3.7'
 nbformat==5.1.3; python_version >= '3.5'
 nest-asyncio==1.5.4; python_version >= '3.5'
 notebook==6.1.6; python_version >= '3.5'