Try to fix Travis with PyPy

Author: Cito

.travis.yml

@@ -14,7 +14,7 @@ matrix:
        dist: xenial
        sudo: true # required workaround for https://github.com/travis-ci/travis-ci/issues/9815
     -  env: TOXENV=pypy
-       python: pypy-5.7.1
+       python: pypy
     -  env: TOXENV=pre-commit
        python: 3.6
     -  env: TOXENV=mypy