fix verbose assertions from AssertErrorMixin

[fawkesley]

back in 9c483376e4a562bbf0102a7b562695476fbd4699 I factored out
AssertErrorMixin, allowing tests to call self._assert_error(..)

Unfortunately this broke pytest's nice verbose assertion messages so
they look like this:

    def _assert_error(self, response, expected_status, expected_detail):
>       assert expected_status == response.status_code
E       AssertionError

this change means pytest knows to include AssertErrorMixin when it's
rewriting assertion code, so it looks like this again:

    def _assert_error(self, response, expected_status, expected_detail):
>       assert expected_status == response.status_code
E       AssertionError: assert 403 == 200
E        +  where 200 = <Response status_code=200,
"application/json">.status_code

mhep/dev/tests/views/mixins/assert_error.py:3: AssertionError