Update pytest-xdist from 1.16.0 to 1.20.1

requirements.txt

 pyparsing==2.2.0
 pytest==3.2.3
 pytest-cov==2.5.1
-pytest-xdist==1.16.0
+pytest-xdist==1.20.1
 PyYAML==3.12
 six==1.10.0
 typing==3.6.1