Bump matplotlib from 3.8.0 to 3.8.1 in /requirements (AtsushiSakai#931)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.8.0 to 3.8.1.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.8.0...v3.8.1)

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

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

Author: dependabot[bot]

requirements/requirements.txt

@@ -1,6 +1,6 @@
 numpy == 1.26.1
 scipy == 1.11.3
-matplotlib == 3.8.0
+matplotlib == 3.8.1
 cvxpy == 1.4.1
 pytest == 7.4.3 # For unit test
 pytest-xdist == 3.3.1 # For unit test