Update nbsphinx requirement from >=0.3.1 to >=0.9.8

Updates the requirements on [nbsphinx](https://github.com/spatialaudio/nbsphinx) to permit the latest version.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.3.1...0.9.8)

---
updated-dependencies:
- dependency-name: nbsphinx
  dependency-version: 0.9.8
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -18,7 +18,7 @@ sphinx-copybutton
 sphinx-prompt
 sphinx-mdinclude
 numpydoc
-nbsphinx>=0.3.1
+nbsphinx>=0.9.8
 
 importlib-metadata
 prettytable