Bump flake8 from 3.8.4 to 3.9.0

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

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

Author: dependabot-preview[bot]

poetry.lock

@@ -11,7 +11,7 @@ algoliasearch = ">=2.0,<3.0"
 alembic = "1.5.3"
 bandit = "1.5.1"
 click = "7.1.2"
-flake8 = "3.8.4"
+flake8 = "3.9.0"
 flask = "1.1.2"
 Flask-Cors = "3.0.10"
 Flask-Migrate = "2.7.0"