Upgrade pytest to 3.6.0

https://github.com/pytest-dev/pytest/issues/1875 made it impossible to declare
a function as a fixture multiple times, which we were doing across different
modules. The fix was to move our @pytest.fixture calls into decorators in the
tests/fixtures.py module.

tests/test_plugins.py

@@ -1,11 +1,9 @@
 from bs4 import BeautifulSoup as Soup
-from .fixtures import (
+from .fixtures import ( # noqa
     app_client,
 )
 import pytest
 
-pytest.fixture(scope='session')(app_client)
-
 
 def test_plugins_dir_plugin(app_client):
     response = app_client.get(