Drop support to old pytest versions (<3.4)

Let's take the opportunity to reduce the build matrix and test with latest
pytest versions as well

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{30,31,32,33}
-  py{27,36}-pytest{30,31,32,33}-pexpect
+  py{27,34,35,36}-pytest{34,35,36}
+  py{27,36}-pytest36-pexpect
   py{27,36}-pytest{master,features}
   flakes
   readme
@@ -15,10 +15,9 @@ deps =
   pycmd
   # to avoid .eggs
   setuptools_scm
-  pytest30: pytest~=3.0.5
-  pytest31: pytest~=3.1.0
-  pytest32: pytest~=3.2.0
-  pytest33: pytest~=3.3.0
+  pytest34: pytest~=3.4.0
+  pytest35: pytest~=3.5.0
+  pytest36: pytest~=3.6.0
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
   pytestfeatures: git+https://github.com/pytest-dev/pytest.git@features
   pexpect: pexpect