Update matplotlib requirement from ~=3.8.4 to ~=3.9.1

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@v3.8.4...v3.9.1)

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

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

requirements.txt

@@ -1,4 +1,4 @@
-matplotlib~=3.8.4
+matplotlib~=3.9.1
 numpy~=1.26.4
 scipy~=1.13.1
 pandas~=2.2.2