Bump flake8 from 3.8.2 to 3.8.3 in /backend

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.2 to 3.8.3.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.2...3.8.3)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -20,7 +20,7 @@ cryptography==2.9.2
 
 flask_jwt_extended==3.24.1
 
-flake8==3.8.2
+flake8==3.8.3
 black==19.10b0
 pre-commit==2.4.0