Bump ruff from 0.0.280 to 0.0.281 in /requirements (AtsushiSakai#883)

Bumps [ruff](https://github.com/astral-sh/ruff) from 0.0.280 to 0.0.281.
- [Release notes](https://github.com/astral-sh/ruff/releases)
- [Changelog](https://github.com/astral-sh/ruff/blob/main/BREAKING_CHANGES.md)
- [Commits](astral-sh/ruff@v0.0.280...v0.0.281)

---
updated-dependencies:
- dependency-name: ruff
  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

@@ -5,4 +5,4 @@ cvxpy == 1.3.2
 pytest == 7.4.0 # For unit test
 pytest-xdist == 3.3.1 # For unit test
 mypy == 1.4.1 # For unit test
-ruff == 0.0.280 # For unit test
+ruff == 0.0.281 # For unit test