Enable Python 3.8-dev testing on Travis

Author: nicoddemus

.travis.yml

@@ -1,4 +1,5 @@
 language: python
+dist: xenial
 
 install:
   - pip install -U pip
@@ -27,8 +28,8 @@ jobs:
       env: TOXENV=py36
     - python: '3.7'
       env: TOXENV=py37
-      sudo: required
-      dist: xenial
+    - python: '3.8-dev'
+      env: TOXENV=py38
     - python: '3.6'
       env: TOXENV=linting
     - python: '3.6'