Test via tox under both Python2 and Python3

MANIFEST.in

+include tox.ini
 recursive-include golang/testdata *.py

setup.py

@@ -28,6 +28,8 @@ setup(
 
     classifiers = [_.strip() for _ in """\
         Development Status :: 3 - Alpha
-        Intended Audience :: Developers\
+        Intended Audience :: Developers
+        Programming Language :: Python :: 2
+        Programming Language :: Python :: 3\
     """.splitlines()]
 )

tox.ini

+[tox]
+envlist = py27, py36, py37
+
+[testenv]
+deps    =
+    # why tox does not get it from extras_require['test'] ?
+    pytest
+
+commands= {envpython} -m pytest