Run tests on macOS. Fixes #446. (#451)

* Run tests on macOS. Fixes #446.

* Force the link in case it exists.

Author: jaraco

.travis-osx

@@ -0,0 +1,6 @@
+# Perform the manual steps on osx to install python3 and activate an environment
+brew update
+brew install python3
+rm /usr/local/bin/python
+ln -s python3 /usr/local/bin/python
+ln -fs pip3 /usr/local/bin/pip

.travis.yml

@@ -16,6 +16,11 @@ matrix:
   - python: 3.7-dev
   - python: nightly
   - python: pypy
+  include:
+  - os: osx
+    language: generic
 cache: python
+before_install:
+- if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then source .travis-osx; fi
 install: pip install tox
 script: tox -e py