Pin to Pytest 3.0.2 as it appears 3.0.3 re-introduces pytest-dev/pytest#1888.

setup.py

@@ -181,7 +181,8 @@ setup_params = dict(
     tests_require=[
         'setuptools[ssl]',
         'pytest-flake8',
-        'pytest>=3.0.2',
+        # pin pytest to 3.0.2 for pytest-dev/pytest#1888
+        'pytest==3.0.2',
     ] + (['mock'] if sys.version_info[:2] < (3, 3) else []),
     setup_requires=[
     ] + pytest_runner + wheel,