Fix version of mypy in Travis test

Cito · Cito · commit bbbe880673e2 · 2019-10-11T21:24:15.000+02:00
diff --git a/tox.ini b/tox.ini
@@ -25,7 +25,7 @@ commands =
 [testenv:mypy]
 basepython=python3.6
 deps =
-    mypy
+    mypy==0.720
 commands =
     mypy graphql --ignore-missing-imports
 
