CI: Use matrix include for Python 3.7

Author: effigies

.travis.yml

@@ -9,8 +9,6 @@ language: python
 # Run jobs on container-based infrastructure, can be overridden per job
 sudo: false
 
-dist: xenial
-
 cache:
   directories:
     - $HOME/.cache/pip
@@ -27,7 +25,6 @@ python:
     - 3.4
     - 3.5
     - 3.6
-    - 3.7
 matrix:
   include:
     - python: 2.7
@@ -83,6 +80,8 @@ matrix:
     - python: 3.5
       env:
         - DOC_DOC_TEST=1
+    - python: 3.7
+      dist: xenial
     # Run tests with indexed_gzip present
     - python: 2.7
       env: