add osx to Travis (#202)

* add CI macOS

* add CI Windows

* update CI

* drop Win

* update CI

* update CI

.travis.yml

@@ -8,8 +8,6 @@
 # this file is *not* meant to cover or endorse the use of travis, but rather to
 # help confirm pull requests to this project.
 
-dist: xenial  # Ubuntu 16.04
-
 env:
   global:
     - DISPLAY=""
@@ -18,10 +16,36 @@ language: python
 
 matrix:
   include:
-    - python: 3.6
+    - os: linux
+      dist: xenial  # Ubuntu 16.04
+      python: 3.6
       env: TOXENV=py36
-    - python: 3.7
+    - os: linux
+      dist: bionic  # Ubuntu 18.04
+      python: 3.6
+      env: TOXENV=py36
+    - os: linux
+      dist: bionic  # Ubuntu 18.04
+      python: 3.7
       env: TOXENV=py37
+    - os: osx
+      osx_image: xcode9.4
+      language: generic
+      env: TOXENV=py36
+      addons:
+        homebrew:
+          # update: true
+          packages: python3
+      before_install:
+        - pip3 install virtualenv
+        - virtualenv -p python3 ~/venv
+        - source ~/venv/bin/activate
+#    - os: windows
+#      language: minimal
+#      before_install:
+#        - choco install python3
+#        - export PATH="/c/Python37:/c/Python37/Scripts:$PATH"
+#      env: TOXENV=py37
 
 # See http://docs.travis-ci.com/user/caching/#pip-cache
 cache: pip