Update pytest-xdist from 1.16.0 to 1.18.0

requirements.txt

@@ -9,7 +9,7 @@ pytest==3.0.7
 pytest-django==3.1.2
 pytest-flake8==0.8.1
 pytest-mock==1.6.0
-pytest-xdist==1.16.0
+pytest-xdist==1.18.0
 factory-boy==2.8.1
 mock==2.0.0