switch Travis back to 3.7-dev

Also use older Sphinx for py2.6

.travis.yml

@@ -1,18 +1,51 @@
 language: python
 cache: pip
-python:
-# temporarily disabled due to broken pysmi dependency
-  - "2.6"
-  - "2.7"
-  - "3.2"
-  - "3.3"
-  - "3.4"
-  - "3.5"
-  - "3.6"
-  - "3.7"
-  - "nightly"
-#  - "pypy"
-#  - "pypy3"
+matrix:
+  include:
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '2.6'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '2.7'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '3.2'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '3.3'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '3.4'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '3.5'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '3.6'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: '3.7-dev'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: 'nightly'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: 'pypy'
+  - os: linux
+    dist: xenial
+    sudo: false
+    python: 'pypy3'
 install:
   - pip install -r requirements.txt -r devel-requirements.txt
   - pip install -e .

devel-requirements.txt

@@ -1 +1,2 @@
-sphinx
+Sphinx <= 1.6; python_version < '2.7'
+Sphinx > 1.6; python_version >= '2.7'