Allow tox to run mypy type checker

I only enabled mypy for python 3.3 and above, since I don't think
there's a python 2 version. (The python 3 version can check python 2
types, but I'm not sure how to do that within tox.)

tox.ini

@@ -1,11 +1,15 @@
 [tox]
-envlist = py{26,27,py,33,34,35}-{test,stylecheck}
+envlist = py{26,27,py,33,34,35}-{test,stylecheck},py{33,34,35}-typecheck
 
 [testenv]
 deps =
     test: pytest
     stylecheck: flake8
     stylecheck: flake8-import-order
+    typecheck: mypy-lang
+    typecheck: typed-ast
+    typecheck: typing
 commands =
     test: py.test []
     stylecheck: flake8 []
+    typecheck: mypy atomicwrites