tox: add mypy environment

tox.ini

 [tox]
-envlist=flake8,py3
+envlist=flake8,mypy,py3
 
 [testenv:py3]
 deps =
@@ -29,3 +29,11 @@ deps =
 commands =
   {envpython} -m flake8 \
     --exclude=.tox,.git,__pycache__,.tox,.eggs,*.egg,swh/deposit/migrations
+
+[testenv:mypy]
+skip_install = true
+deps =
+  .[testing]
+  mypy
+commands =
+  mypy swh