Fix deployment using Python 3.7

The Xenial environment needs to be used. See
https://docs.travis-ci.com/user/languages/python/#python-37-and-higher
for further information.

Author: SkypLabs

.travis.yml

@@ -17,6 +17,7 @@ jobs:
       dist: xenial
     - stage: PyPI release
       python: 3.7
+      dist: xenial
       deploy:
         provider: pypi
         distributions: sdist