Update pytest-xdist from 1.16.0 to 1.17.1

tests/ui/requirements.txt

@@ -5,5 +5,5 @@ pytest==3.1.2
 pytest-instafail==0.3.0
 pytest-selenium==1.10.0
 pytest-variables==1.6.1
-pytest-xdist==1.16.0
+pytest-xdist==1.17.1
 selenium==3.4.3