Bump nbconvert from 4.0.0 to 6.3.0

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 4.0.0 to 6.3.0.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](https://github.com/jupyter/nbconvert/compare/4.0.0...6.3.0)

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

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

requirements.txt

@@ -19,7 +19,7 @@ MarkupSafe==0.23
 matplotlib==1.4.3
 mistune==2.0.3
 mock==1.3.0
-nbconvert==4.0.0
+nbconvert==6.3.0
 nbformat==4.0.0
 nose==1.3.7
 notebook==4.0.4