Travis: add test for windows + osx (test)

.travis.yml

@@ -1,12 +1,26 @@
-language: python
-python:
-    - 3.6
-    - 3.7
-    - 3.8
-before_install:
-    - python --version
-    - pip install -U pip
-install:
-    - pip install ".[dev]" .
-script:
-    - python -m unittest discover -s tests -t .
+language: python      # this works for Linux but is an error on macOS or Windows
+jobs:
+  include:
+    - name: "Python 3.6.0 on Xenial Linux"
+      python: 3.6
+    - name: "Python 3.7.0 on Xenial Linux"
+      python: 3.7
+    - name: "Python 3.8.0 on Xenial Linux"
+      python: 3.8     # this works for Linux but is ignored on macOS or Windows
+    - name: "Python 3.7.4 on macOS"
+      os: osx
+      osx_image: xcode11.2  # Python 3.7.4 running on macOS 10.14.4
+      language: shell       # 'language: python' is an error on Travis CI macOS
+    - name: "Python 3.8.0 on Windows"
+      os: windows           # Windows 10.0.17134 N/A Build 17134
+      language: shell       # 'language: python' is an error on Travis CI Windows
+      before_install:
+        - choco install python --version 3.8.0
+        - python -m pip install --upgrade pip
+      env: PATH=/c/Python38:/c/Python38/Scripts:$PATH
+install: 
+    - pip3 install --upgrade pip  # all three OSes agree about 'pip3'
+    - pip3 install ".[dev]" .
+# 'python' points to Python 2.7 on macOS but points to Python 3.8 on Linux and Windows
+# 'python3' is a 'command not found' error on Windows but 'py' works on Windows only
+script: python -m unittest discover -s tests -t . || python3 -m unittest discover -s tests -t .