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

@@ -3,5 +3,5 @@ pytest==6.2.5
 pytest-cov==3.0.0
 pycodestyle==2.7.0
 pydocstyle==6.1.1
-flake8==3.9.2
+flake8==4.0.1
 mypy==0.812