Bump flake8 from 3.8.4 to 3.9.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.1)

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

requirements.txt

@@ -17,7 +17,7 @@ uWSGI>=2.0.19,<2.1
 whitenoise>=5.2,<5.3
 
 # Linting
-flake8==3.8.4
+flake8==3.9.1
 flake8-debugger==4.0.0
 isort==5.7.0