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

@@ -31,7 +31,7 @@ jupyter-client==6.1.3
 jupyter-core==4.6.3
 markupsafe==1.1.1
 mistune==0.8.4
-nbconvert==5.6.1
+nbconvert==6.5.1
 nbformat==5.0.6
 notebook==6.0.3
 numpy==1.18.4