Update matplotlib requirement from <3.8 to <3.9

Updates the requirements on [matplotlib](https://github.com/matplotlib/matplotlib) to permit the latest version.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v0.91.2...v3.8.0)

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

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

docs/notebook-requirements.txt

@@ -8,5 +8,5 @@ nbsphinx>=0.8.6,<0.8.7
 Jinja2<3.1
 
 # Dependencies
-matplotlib<3.8
+matplotlib<3.9
 pytest