Add mypy env

There are still kinks to be worked out but this works reasonably well.
This will fail until we can figure out how to import things from the
typing module for the type: comments only. We do not want to add a
dependency on that backport only for this mypy env.

tox.ini

@@ -35,6 +35,16 @@ deps =
 commands =
     pylint flake8
 
+[testenv:mypy]
+basepython = python3
+skipsdist = true
+skip_install = true
+use_develop = false
+deps =
+    mypy-lang
+commands =
+    mypy flake8
+
 [testenv:docs]
 deps =
     sphinx>=1.3.0