Add python3.8 and python3.9-dev to travis-ci (#73)

* Add python38 to travis

* Add python39-dev

.travis.yml

@@ -1,5 +1,5 @@
 language: python
-dist: xenial
+dist: bionic
 sudo: false
 cache: false
 stages:
@@ -18,15 +18,14 @@ script:
 jobs:
   include:
     - python: "2.7"
-
     - python: "3.5"
-
     - python: "3.6"
-
     - python: "3.7"
+    - python: "3.8"
+    - python: "3.9-dev"
 
     - stage: deploy
-      python: "3.7"
+      python: "3.8"
       deploy:
         on:
           repo: davegallant/rfd