Update flake8 requirement from <5 to <7

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@0.1...6.0.0)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements-test.txt

@@ -8,7 +8,7 @@
 # testing tools
 bandit
 coverage
-flake8<5
+flake8<7
 pytest<8
 pytest-cov
 pytest-flake8