ci: Travis: use Python 3.8 instead of 3.8-dev (pytest-dev#792)

Author: blueyed

.travis.yml

@@ -26,7 +26,7 @@ jobs:
       env: TOXENV=py37-dj21-sqlite-coverage
     - python: 3.7
       env: TOXENV=py37-dj22-sqlite-xdist-coverage
-    - python: 3.8-dev
+    - python: 3.8
       env: TOXENV=py38-dj30-sqlite-xdist-coverage
 
     # Explicitly test (older) pytest 4.1.