Update nbsphinx requirement from ~=0.8 to ~=0.9

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@0.8.0...0.9.6)

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

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

docs/requirements.txt

@@ -5,7 +5,7 @@ recommonmark ~=0.7
 pygments-style-monokailight ~=0.4
 ipython ~=7.19
 pygments ~=2.4
-nbsphinx ~=0.8
+nbsphinx ~=0.9
 sphinxcontrib-jquery ~=4.1
 sphinx-design
 pydata-sphinx-theme