Update tested Python versions on Travis

Author: acdha

.travis.yml

@@ -1,12 +1,10 @@
 language: python
 python:
  - "2.7"
- - "3.3"
- - "3.4"
- - "3.5"
  - "3.6"
- - "3.6-dev"
+ - "3.7"
  - "3.7-dev"
+ - "3.8-dev"
 
 addons:
   apt:
@@ -16,8 +14,8 @@ addons:
 matrix:
   allow_failures:
     python:
-        - "3.6-dev"
         - "3.7-dev"
+        - "3.8-dev"
 
 install:
     - pip install --upgrade pip