Bump flake8 from 3.8.3 to 3.8.4

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.3 to 3.8.4.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.3...3.8.4)

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

requirements.txt

 whitenoise==5.2.0
 
 # Linting
-flake8==3.8.3
+flake8==3.8.4
 flake8-debugger==3.2.1
 flake8-blind-except==0.1.1
 isort==5.6.4