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.

setup.py

@@ -48,7 +48,7 @@ setup(
     ''',
     setup_requires=['pytest-runner'],
     tests_require=[
-        'pytest==3.2.1',
+        'pytest==3.6.0',
         'aiohttp==2.3.2',
         'beautifulsoup4==4.6.0',
     ],