Travis CI: Add Python 3.8 to the testing

Author: cclauss

.travis.yml

@@ -8,9 +8,6 @@ cache:
 python:
     - 3.6
 
-sudo: required
-
-
 env:
   global:
     - PATH=$TRAVIS_BUILD_DIR/pandoc:$PATH
@@ -64,7 +61,8 @@ matrix:
           env: GROUP=python
         - python: 3.7
           env: GROUP=python
-          dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+        - python: 3.8
+          env: GROUP=python
         - python: 3.6
           env: GROUP=docs