Tox for cross Python-version testing

[nils-werner]

Note that the way Travis CI and py.test run their tests remain unchanged:

• Travis CI runs multiple Python version tests in seprarate images
• py.test works exactly as before

The only difference: Users can run tox on their local machines to test Python 2 and 3.

[hameerabbasi]

Can you also move the py.test options from setup.cfg to pytest.ini, since it's deprecated in 3.x? (I can't find a link in the docs, but see pytest-dev/pytest#3086)

[nils-werner]

The way I understood it was [pytest] was deprecated in favor of [tool:pytest], because the values from [whatever] where picked up when you run python setup.py whatever

[hameerabbasi]

Ah, in that case... We should be good.

[nils-werner]

Related to this issue and #71 it seems that NumPy<=1.12.0 has some issues that are not present in NumPy latest. It may make sense to test for multiple Python versions and multiple NumPy versions:

[tox]
envlist = {py27,py36}-{numpy12,numpylatest}
[testenv]
deps =
   numpy12: numpy==1.12.0
basepython =
   py27: python2.7
   py36: python3.6
commands=
   pip list
   py.test {posargs}
extras=
   docs
   tests