Decouple mypy from tox

This allows mypy to produce the same results regardless how is called.
For example IDEs will produce the same results as tox -e linters
instead of displaying violations which the core team decided to
disable.

Change-Id: I20eeafc0549f78e3d4f9636793cfe031389b6bbe

setup.cfg

@@ -61,3 +61,6 @@ zuul_base =
 
 [mypy]
 allow_redefinition = True
+files = zuul
+ignore_missing_imports = True
+python_version = 3.6

tox.ini

@@ -59,7 +59,7 @@ deps =
   openapi-spec-validator
 commands =
   flake8 {posargs}
-  mypy --ignore-missing-imports zuul
+  mypy
   openapi-spec-validator web/public/openapi.yaml
 
 [testenv:cover]