docs: update supported python versions

.travis.yml

@@ -1,9 +1,13 @@
 # Enable container based builds
 sudo: false
+dist: xenial   # required for Python >= 3.7
 language: python
 
 matrix:
   include:
+    - python: 3.7
+      env:
+        - TOXENV=py37
     - python: 3.6
       env:
         - TOXENV=py36

DEVELOPMENT_GUIDE.rst

@@ -17,10 +17,8 @@ steps manually.
 
 1. Install Python Versions
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
-Python 2.7 is our officially supported Python version. We have a future goal to support to Python3.6 but the code base
-currently does not work with Python3. To make future migration easier, any new code we write must be compatible with
-Python3. Follow the idioms from this `excellent cheatsheet`_ to make sure your code is compatible with both Python
-versions.
+Our officially supported Python versions are 2.7, 3.6, and 3.7. Follow the idioms from this `excellent cheatsheet`_ to
+make sure your code is compatible with both Python 2.7 and 3 versions.
 
 Setup Python locally using `pyenv`_