only run travis on 3.6 and up, and drop the special xenial 3.7 thingy

Author: radix

.travis.yml

@@ -2,15 +2,10 @@ sudo: false
 
 language: python
 python:
-    - "2.7"
-    - "3.4"
-    - "3.5"
+    - "3.6"
+    - "3.7"
+    - "3.8"
     - "pypy"
-matrix:
-    include:
-        - python: 3.7
-          dist: xenial
-          sudo: true
 install:
     - pip install .
     - pip install -r dev-requirements.txt