CI: add python 3.8 env to travis

Author: tacaswell

.travis.yml

@@ -81,6 +81,7 @@ matrix:
         - DELETE_FONT_CACHE=1
         - EXTRAREQS='-r requirements/testing/travis36.txt'
     - python: 3.7
+    - python: 3.8
     - python: "nightly"
       env:
         - PRE=--pre