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:development
...

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

requirements-lock-dev.txt

@@ -65,7 +65,7 @@ matplotlib==3.1.1
 mccabe==0.6.1             # via flake8
 mistune==0.8.4            # via nbconvert
 more-itertools==7.2.0     # via pytest, zipp
-nbconvert==5.6.1          # via ipypublish, notebook
+nbconvert==6.5.1          # via ipypublish, notebook
 nbformat==4.4.0           # via ipypublish, ipywidgets, jupytext, nbconvert, notebook
 nodeenv==1.3.3            # via pre-commit
 notebook==6.0.2           # via widgetsnbextension