chore(deps): update dependency flake8 to v3.9.2

Author: renovate[bot]

requirements.txt

@@ -7,7 +7,7 @@ MarkupSafe==1.1.1
 SQLAlchemy==1.3.16
 Werkzeug==0.16.1
 cloudinary==1.20.0
-flake8==3.8.4
+flake8==3.9.2
 gunicorn==20.0.4
 itsdangerous==1.1
 mccabe==0.6.1