Update mock from 2.0.0 to 3.0.4

Author: pyup-bot

requirements/pip.txt

@@ -68,7 +68,7 @@ django-gravatar2==1.4.2
 pytz==2019.1
 Unipath==1.1
 django-kombu==0.9.4
-mock==2.0.0
+mock==3.0.4
 stripe==2.27.0
 
 # unicode-slugify==0.1.5 is not released on PyPI yet