Update CI to test against latest pytest release

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{30,31,32,33,36,38}
+  py{27,34,35,36}-pytest{30,31,32,33,latest}
   py{27,36}-pytest36-pexpect
   py{27,36}-pytest{master,features}
 
@@ -18,8 +18,7 @@ deps =
   pytest31: pytest~=3.1.0
   pytest32: pytest~=3.2.0
   pytest33: pytest~=3.3.0
-  pytest36: pytest~=3.6.0
-  pytest38: pytest~=3.8.0
+  pytestlatest: pytest
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
   pytestfeatures: git+https://github.com/pytest-dev/pytest.git@features
   pexpect: pexpect
@@ -27,8 +26,6 @@ deps =
 platform=
   pexpect: linux|darwin
 commands=
-  # always clean to avoid code unmarshal mismatch on old python/pytest
-  py.cleanup -aq
   pytest {posargs}
 
 [testenv:linting]