pull in dev deps for tox mypy job

It's going to work this way after poetry anyway

Author: David Robertson

tox.ini

@@ -151,5 +151,5 @@ commands = isort -c --df {[base]lint_targets}
 [testenv:mypy]
 deps =
     {[base]deps}
-extras = all,mypy
+extras = all,mypy,dev
 commands = mypy