Update pytest-runner from 3.0 to 4.0

Author: pyup-bot

requirements_dev.txt

@@ -39,7 +39,7 @@ pycparser==2.18
 pyflakes==1.6.0
 Pygments==2.2.0
 pytest==3.3.2
-pytest-runner==3.0
+pytest-runner==4.0
 python-dateutil==2.6.1
 pytz==2017.3
 PyYAML==3.12