Add pytest 3.3 to tox build matrix

tox.ini

@@ -1,8 +1,8 @@
 [tox]
 # if you change the envlist, please update .travis.yml file as well
 envlist=
-  py{27,34,35,36}-pytest{30,31,32}
-  py{27,36}-pytest{30,31,32}-pexpect
+  py{27,34,35,36}-pytest{30,31,32,33}
+  py{27,36}-pytest{30,31,32,33}-pexpect
   py{27,36}-pytest{master,features}
   flakes
   readme
@@ -18,6 +18,7 @@ deps =
   pytest30: pytest~=3.0.5
   pytest31: pytest~=3.1.0
   pytest32: pytest~=3.2.0
+  pytest33: pytest~=3.3.0
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
   pytestfeatures: git+https://github.com/pytest-dev/pytest.git@features
   pexpect: pexpect