Extract Python versions as top-level factors.

.travis.yml

 dist: trusty
 sudo: false
 language: python
+python:
+- 2.6
+- &latest_py2 2.7
+- 3.3
+- 3.4
+- 3.5
+- &latest_py3 3.6
+- nightly
+- pypy
+- pypy3
+
 jobs:
   fast_finish: true
   include:
-  - python: 2.6
-  - python: &latest_py2 2.7
-  - python: 3.3
-  - python: 3.4
-  - python: 3.5
-  - python: &latest_py3 3.6
-  - python: nightly
-  - python: pypy
-  - python: pypy3
   - python: *latest_py3
     env: LANG=C
   - python: *latest_py2