Bump nbconvert from 6.3.0 to 6.5.1 in /source

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 6.3.0 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@6.3.0...6.5.1)

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

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

Author: dependabot[bot]

source/rtd_requirements.txt

@@ -12,4 +12,4 @@ matplotlib
 pandas
 pandas-bokeh
 imageio
-nbconvert==6.4.4
+nbconvert==6.5.1