Bump nbconvert from 5.5.0 to 6.3.0

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 5.5.0 to 6.3.0.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@5.5...6.3.0)

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

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Author: dependabot[bot]