enable osx on travis

.travis.yml

@@ -9,26 +9,39 @@ env:
 
 matrix:
   include:
+    # dedicated job to just build the docs
+    - python: "3.6"
+      env: BIDICT_BUILD_DOCS_INSTEAD=1
+
+    # jobs to test on all supported python versions on Linux
     - python: "pypy"
-      env:
-        - PYENV_PYTHON_VER=pypy2.7-portable-5.9.0
+      env: PYENV_PYTHON_VER=pypy2.7-portable-5.9.0
     - python: "pypy3"
-      env:
-        - PYENV_PYTHON_VER=pypy3.5-portable-5.9.0
+      env: PYENV_PYTHON_VER=pypy3.5-portable-5.9.0
     - python: "2.7"
-      env:
-        - PYENV_PYTHON_VER=2.7.14
+      env: PYENV_PYTHON_VER=2.7.14
     - python: "3.5"
-      env:
-        - PYENV_PYTHON_VER=3.5.3
+      env: PYENV_PYTHON_VER=3.5.3
     - python: "3.6"
-      env:
-        - PYENV_PYTHON_VER=3.6.3
-        # Only generate coverage report on the job for latest Python 3
-        - BIDICT_COVERAGE_ENABLE=1
+      # only generate coverage report for this one job
+      env: PYENV_PYTHON_VER=3.6.3 BIDICT_COVERAGE_ENABLE=1
+
+    # test all of the above again on macOS
+    - python: "pypy"
+      env: PYENV_PYTHON_VER=pypy2.7-portable-5.9.0
+      os: osx
+    - python: "pypy3"
+      env: PYENV_PYTHON_VER=pypy3.5-portable-5.9.0
+      os: osx
+    - python: "2.7"
+      env: PYENV_PYTHON_VER=2.7.14
+      os: osx
+    - python: "3.5"
+      env: PYENV_PYTHON_VER=3.5.3
+      os: osx
     - python: "3.6"
-      env:
-        - BIDICT_BUILD_DOCS_INSTEAD=1
+      env: PYENV_PYTHON_VER=3.6.3
+      os: osx
 
 install:
   # based on https://github.com/frol/flask-restplus-server-example/blob/018f48e5/.travis.yml