Bump flake8 from 3.7.9 to 3.8.2 in /backend (#156)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.9 to 3.8.2.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.9...3.8.2)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -20,7 +20,7 @@ cryptography==2.9.2
 
 flask_jwt_extended==3.24.1
 
-flake8==3.7.9
+flake8==3.8.2
 black==19.10b0
 pre-commit==2.4.0