Bump flake8 from 3.8.1 to 3.9.2

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.1 to 3.9.2.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.1...3.9.2)

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

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

requirements/testing.txt

@@ -1,7 +1,7 @@
 -r production.txt
 isort==4.3.21
 coveralls==2.0.0
-flake8==3.8.1
+flake8==3.9.2
 flake8-isort==2.8.0
 pydocstyle==5.0.2
 pylint==2.5.2