tox: do not set basepython at all

Tox handles this good by itself.

Author: blueyed

tox.ini

@@ -8,14 +8,6 @@ envlist =
 
 [testenv]
 extras = testing
-basepython =
-    pypy: {env:TOXPYTHON:pypy}
-    pypy3: {env:TOXPYTHON:pypy3}
-    {py27,docs}: {env:TOXPYTHON:python2.7}
-    py34: {env:TOXPYTHON:python3.4}
-    py35: {env:TOXPYTHON:python3.5}
-    py36: {env:TOXPYTHON:python3.6}
-    py37: {env:TOXPYTHON:python3.7}
 setenv =
     PYTHONPATH={toxinidir}/tests
     PYTHONUNBUFFERED=yes