Workaround https://github.com/travis-ci/travis-ci/issues/6865

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index a3e5fc4..c4c3b19 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,7 @@
     -   env: TOXENV=py36
         python: 3.6
     -   env: TOXENV=pypy
+        python: pypy-5.7.1
 install: pip install coveralls tox
 script: tox
 before_install: