Update mock from 2.0.0 to 3.0.2 (#616)

Author: pyup-bot

requirements-development.txt

@@ -7,7 +7,7 @@ Faker==1.0.5
 flake8==3.7.7
 flake8-isort==2.7.0
 isort==4.3.18
-mock==2.0.0
+mock==3.0.2
 pytest==4.4.1
 pytest-cov==2.6.1
 pytest-django==3.4.8