Workaround for py35

Author: hugovk

.travis.yml

@@ -9,6 +9,10 @@ env:
   - TOX_ENV=py35
   - TOX_ENV=py36
 
+before_install:
+  # work around https://github.com/travis-ci/travis-ci/issues/8363
+  - pyenv global system 3.5
+
 install:
   - travis_retry pip install -U pip wheel tox
   - travis_retry pip install -U -r requirements.txt -e .