Get mypy tox env running in the current setup (#5861)

* get mypy tox env running in the current setup

* use any python3 with mypy

* pin mypy dependencies

tox.ini

@@ -136,9 +136,9 @@ commands =
     pylint --reports=n --rcfile=.pylintrc {[base]source_paths}
 
 [testenv:mypy]
-basepython = python3.4
+basepython = python3
 commands =
-    {[base]pip_install} mypy
+    {[base]pip_install} .[dev3]
     {[base]install_packages}
     mypy --py2 --ignore-missing-imports {[base]source_paths}