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](https://github.com/jupyter/nbconvert/compare/5.6.1...6.5.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -38,7 +38,7 @@ mistune==0.8.4
 mkl-fft==1.0.15
 mkl-random==1.1.0
 mkl-service==2.3.0
-nbconvert==5.6.1
+nbconvert==6.5.1
 nbformat==5.0.5
 networkx==2.4
 notebook==6.0.3