Update pytest from 3.6.1 to 4.2.0

Author: pyup-bot

requirements-dev.txt

@@ -11,7 +11,7 @@ ipdb==0.11
 motor==1.2.2
 pytest-cov==2.5.1
 pytest-sugar==0.9.1
-pytest==3.6.1
+pytest==4.2.0
 python-dateutil==2.7.3
 sqlalchemy==1.2.8
 trafaret==2.0.0a1