Fix warnings transfer between workers and master node with pytest >= 3.8

Fix #341

tox.ini

@@ -2,7 +2,7 @@
 # if you change the envlist, please update .travis.yml file as well
 envlist=
   linting
-  py{27,34,35,36}-pytest{30,31,32,33,36}
+  py{27,34,35,36}-pytest{30,31,32,33,36,38}
   py{27,36}-pytest36-pexpect
   py{27,36}-pytest{master,features}
 
@@ -19,6 +19,7 @@ deps =
   pytest32: pytest~=3.2.0
   pytest33: pytest~=3.3.0
   pytest36: pytest~=3.6.0
+  pytest38: pytest~=3.8.0
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
   pytestfeatures: git+https://github.com/pytest-dev/pytest.git@features
   pexpect: pexpect