Add Python 3.7 support and testing

Python 3.7 final has been released [0]. Re-add this to continuous integration testing, and
note the support level in setup.py for PyPI and other packagers.

Note: Travis-CI's support for Python 3.7 and communication around it is less than ideal.

A Travis-CI representative recently provided comment on their "official" workaround [1],
for users who need to meet Python 3.7's dependencies, yet aren't ready to move their entire
testing image to Ubuntu Xenial (16.04) from default Ubuntu Trusty (14.04) at this stage.
See further comments at [2].

[0] https://www.python.org/downloads/release/python-370/
[1] travis-ci/travis-ci#9069 (comment)
[2] travis-ci/travis-ci#9815

Author: Echelon9

.travis.yml

@@ -1,8 +1,15 @@
 language: python
+# Existing Python versions
 python:
   - "2.7"
   - "3.5"
   - "3.6"
+# Enable 3.7 without globally enabling sudo and dist: xenial for other build jobs
+matrix:
+  include:
+    - python: "3.7"
+      dist: xenial
+      sudo: required
 before_install: pip install pipenv
 install: pipenv install
 script:

setup.py

@@ -19,6 +19,7 @@
         'six'
     ],
     classifiers=(
+        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.4",