Roll back to mypy 0.521 to avoid a bug in 0.530.

Filed here: https://github.com/python/mypy/issues/4069

We should go back to tracking the latest mypy releases as soon as
possible.

.travis.yml

@@ -9,7 +9,7 @@ addons:
         packages:
             - python3-pip
 install:
-    - pip3 install --user --upgrade mypy flake8
+    - pip3 install --user --upgrade mypy==0.521 flake8
     - travis_wait ./check-rustfmt.sh --install
 script: ./test-all.sh
 cache: