The issue that you are having is unrelated to the fact that this is a Celery task. You just happen to be patching the wrong thing. 😉
Specifically, you need to find out which view or other file is importing “mytask” and patch it over there, so the relevant line would look like this:
with patch('myapp.myview.mytask.delay') as mock_task:
There is some more flavor to this here:
http://www.voidspace.org.uk/python/mock/patch.html#where-to-patch