build(deps): bump cvxpy from 1.8.1 to 1.9.1 in /requirements

Bumps [cvxpy](https://github.com/cvxpy/cvxpy) from 1.8.1 to 1.9.1.
- [Release notes](https://github.com/cvxpy/cvxpy/releases)
- [Commits](cvxpy/cvxpy@v1.8.1...v1.9.1)

---
updated-dependencies:
- dependency-name: cvxpy
  dependency-version: 1.9.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements/requirements.txt

@@ -1,7 +1,7 @@
 numpy == 2.3.5
 scipy == 1.17.1
 matplotlib == 3.10.8
-cvxpy == 1.8.1
+cvxpy == 1.9.1
 ecos == 2.0.14
 pytest == 9.0.3 # For unit test
 pytest-xdist == 3.8.0 # For unit test