Use Python 3.9 release in Travis configuration

Author: jdufresne

.travis.yml

@@ -21,8 +21,8 @@ matrix:
       - { python: "3.8", env: DJANGO=3.1 }
       - { python: "3.8", env: DJANGO=master }
 
-      - { python: "3.9-dev", env: DJANGO=3.1 }
-      - { python: "3.9-dev", env: DJANGO=master }
+      - { python: "3.9", env: DJANGO=3.1 }
+      - { python: "3.9", env: DJANGO=master }
 
       - { python: "3.8", env: TOXENV=base }
       - { python: "3.8", env: TOXENV=lint }