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

dev-env-requirements.txt

@@ -4,6 +4,6 @@ graphene-django==3.0.0b7
 pytest==4.6.3
 pytest-django==3.5.0
 pytest-cov==2.7.1
-flake8==3.7.7
+flake8==3.9.2
 mock==2.0.0
 black==21.6b0