Bump nbconvert from 6.1.0 to 6.3.0

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 6.1.0 to 6.3.0.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@6.1.0...6.3.0)

---
updated-dependencies:
- dependency-name: nbconvert
  dependency-type: indirect
...

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