.travis.yml: experiment with Python versions #3

Author: davidmalcolm

.travis.yml

@@ -64,13 +64,13 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
 
-  python:
-    - "2.7"
-    - "3.3"
-    - "3.4"
-    - "3.5"
-    - "3.6"
-    - "3.7-dev"  # 3.7 development branch
+  include:
+    - python: "2.7"
+    - python: "3.3"
+    - python: "3.4"
+    - python: "3.5"
+    - python: "3.6"
+    - python: "3.7-dev"  # 3.7 development branch
 
 language: c
 compiler: