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

tools/dev_constraints.txt

@@ -30,6 +30,7 @@ josepy==1.0.1
 logger==1.4
 logilab-common==1.4.1
 MarkupSafe==1.0
+mypy==0.580
 ndg-httpsclient==0.3.2
 oauth2client==2.0.0
 pathlib2==2.3.0
@@ -66,6 +67,8 @@ tox==2.9.1
 tqdm==4.19.4
 traitlets==4.3.2
 twine==1.9.1
+typed-ast==1.1.0
+typing==3.6.4
 uritemplate==0.6
 virtualenv==15.1.0
 wcwidth==0.1.7