[travis] Add python 3.8 build

.travis.yml

@@ -21,6 +21,12 @@ matrix:
     - python: 3.7
       dist: xenial
       env: YTDL_TEST_SET=download
+    - python: 3.8
+      dist: xenial
+      env: YTDL_TEST_SET=core
+    - python: 3.8
+      dist: xenial
+      env: YTDL_TEST_SET=download
     - python: 3.8-dev
       dist: xenial
       env: YTDL_TEST_SET=core