CI: Run tests on Python 3.8

Python 3.8 was released in October 2019.

Author: adrienverge

.travis.yml

@@ -6,6 +6,7 @@ python:
   - 3.5
   - 3.6
   - 3.7
+  - 3.8
   - nightly
 install:
   - pip install pyyaml coveralls flake8 flake8-import-order doc8