update tox config: remove obsolete Python3 versions and allow pytest args

Author: Sara Safavi

tox.ini

@@ -4,10 +4,10 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, py34, py35, py36, py37
+envlist = py27, py36, py37
 
 [testenv]
 deps = pytest
 commands = 
     pip install -e .[dev]
-    pytest
+    pytest {posargs}