travis-ci: tie mypy to version 0.770

Avoid using the latest mypy on each travis-ci run, as an upgraded
mypy may have new checks that cause failures unrelated to a current
PR. Hold the version at the last known good version: mypy-0.770.

.travis.yml

@@ -29,7 +29,7 @@ jobs:
       name: 'mypy'
       language: 'python'
       python: '3.6'
-      install: python3 -m pip install 'mypy' --force-reinstall
+      install: python3 -m pip install 'mypy==0.770' --force-reinstall
       script: mypy
       before_deploy: skip
       deploy: skip