[.travis.yml] transition to mypy static type checking

Author: chrissimpkins

.travis.yml

@@ -10,7 +10,7 @@ matrix:
       dist: xenial
       env: TOX_ENV=static-type-checks
       install:
-        - pip install --upgrade pytype
+        - pip install --upgrade mypy
         - pip install -r requirements.txt
       script: make test-type-check
     - python: 3.7