Add Python 3.7 to Travis CI tests

.travis.yml

@@ -1,16 +1,26 @@
 language: python
-python:
-  - "2.7"
-  - "pypy"
-  - "3.2"
-  - "3.3"
-  - "3.4"
-  - "3.5"
-  - "3.6"
-  - "nightly"
-  - "pypy3"
 
 sudo: false
 
+.mixins:
+  - &xenial
+    dist: xenial
+
+jobs:
+  include:
+    - python: "2.7"
+    - python: "pypy"
+    - python: "3.2"
+    - python: "3.3"
+    - python: "3.4"
+    - python: "3.5"
+    - python: "3.6"
+    - <<: *xenial
+      python: "3.7"
+    - <<: *xenial
+      python: "3.8-dev"
+    - python: "nightly"
+    - python: "pypy3"
+
 script:
   - ./test/test_travis