Pytest Timezone Warning Fixed

[saadmk11]

While running tox -e py36 was getting fludded with pytest warnings like this:

readthedocs/api/v3/tests/test_projects.py::APIEndpointTests::test_unauthed_others_projects_detail
  /home/saad/dev/readthedocs.org/.tox/py36/lib/python3.6/site-packages/django/db/models/fields/__init__.py:1451: RuntimeWarning: DateTimeField User.date_joined received a naive datetime (2019-04-29 10:00:00) while time zone support is active.
    RuntimeWarning)
  /home/saad/dev/readthedocs.org/.tox/py36/lib/python3.6/site-packages/django/db/models/fields/__init__.py:1451: RuntimeWarning: DateTimeField Project.pub_date received a naive datetime (2019-04-29 10:00:00) while time zone support is active.
    RuntimeWarning)
  /home/saad/dev/readthedocs.org/.tox/py36/lib/python3.6/site-packages/django/db/models/fields/__init__.py:1451: RuntimeWarning: DateTimeField Project.modified_date received a naive datetime (2019-04-29 12:00:00) while time zone support is active.
    RuntimeWarning)
  /home/saad/dev/readthedocs.org/.tox/py36/lib/python3.6/site-packages/django/db/models/fields/__init__.py:1451: RuntimeWarning: DateTimeField Project.pub_date received a naive datetime (2019-04-29 10:00:00) while time zone support is active.
    RuntimeWarning)
  /home/saad/dev/readthedocs.org/.tox/py36/lib/python3.6/site-packages/django/db/models/fields/__init__.py:1451: RuntimeWarning: DateTimeField Project.modified_date received a naive datetime (2019-04-29 12:00:00) while time zone support is active.

made the datetime objects timezone aware to avoid getting these warnings.

[ericholscher]

Simple enough 👍