Bump flake8 from 3.9.2 to 4.0.1

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.9.2...4.0.1)

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

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

requirements/testing.txt

@@ -1,7 +1,7 @@
 -r production.txt
 isort==5.9.3
 coveralls==3.2.0
-flake8==3.9.2
+flake8==4.0.1
 flake8-isort==4.0.0
 pydocstyle==6.1.1
 pylint==2.11.1