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] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -127,7 +127,7 @@ msgpack==0.6.2
 multidict==4.7.5
 munch==2.5.0
 murmurhash==1.0.2
-nbconvert==5.6.1
+nbconvert==6.5.1
 nbformat==4.4.0
 networkx==2.2
 notebook==6.1.5