v3.0 - remove Python 2.x from build (#983)

* Change travis to only compile for p3.6+

* Changed tox to only run Python 3.6+

* Changed library classifiers to reflect support in Python 3.6+

* Changed version to 3.0.0 development

In [15]: get_version((3, 0, 0, "alpha", 0))
Out[15]: '3.0.dev20190601212304'

Author: ekampf

tox.ini

@@ -5,10 +5,11 @@ skipsdist = true
 [testenv]
 deps =
     .[test]
+    py{36,37}: pytest-asyncio
 setenv =
      PYTHONPATH = .:{envdir}
 commands =
-    py{36,37}: py.test --cov=graphene graphene examples tests_asyncio {posargs}
+    py{36,37}: py.test --cov=graphene graphene examples tests_asyncio tests_py36 {posargs}
 
 [testenv:pre-commit]
 basepython=python3.6