CI: (deps): Bump matplotlib from 3.8.4 to 3.9.0 in /ci

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.8.4 to 3.9.0.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.8.4...v3.9.0)

---
updated-dependencies:
- dependency-name: matplotlib
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

ci/requirements.txt

@@ -1,4 +1,4 @@
-matplotlib==3.8.4
+matplotlib==3.9.0
 numpy==1.26.4
 pandas==2.2.2
 pooch==1.8.2