Bump flake8 from 5.0.2 to 5.0.4 in /requirements (AtsushiSakai#706)

Bumps [flake8](https://github.com/pycqa/flake8) from 5.0.2 to 5.0.4.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@5.0.2...5.0.4)

---
updated-dependencies:
- dependency-name: flake8
  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

@@ -6,4 +6,4 @@ cvxpy == 1.2.1
 pytest == 7.1.2 # For unit test
 pytest-xdist == 2.5.0 # For unit test
 mypy == 0.971 # For unit test
-flake8 == 5.0.2 # For unit test
+flake8 == 5.0.4 # For unit test