Bump flake8 from 3.9.2 to 6.0.0

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 6.0.0.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.9.2...6.0.0)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

requirements-testing.txt

@@ -1,3 +1,3 @@
-flake8==3.9.2
+flake8==6.0.0
 pytest==6.2.4
 pytest-cov==2.12.1