Bump matplotlib from 3.9.0 to 3.9.1 in /requirements (#1045)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.9.0 to 3.9.1.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](https://github.com/matplotlib/matplotlib/compare/v3.9.0...v3.9.1)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/requirements.txt

@@ -1,6 +1,6 @@
 numpy == 2.0.0
 scipy == 1.14.0
-matplotlib == 3.9.0
+matplotlib == 3.9.1
 cvxpy == 1.5.2
 pytest == 8.2.2 # For unit test
 pytest-xdist == 3.6.1 # For unit test