mypy: fix mysterious build error

Author: hauntsaninja

.travis.yml

@@ -82,6 +82,7 @@ jobs:
 install:
 - pip install -U pip setuptools
 - pip install -U tox==3.9.0
+- pip install -U six>=1.12
 - tox --notest
 
 # This is a big hack and only works because the layout of our directories