[Travis] add testing on Windows and Mac

See #739 (Python 3.8), #619 (Windows)

.travis.yml

@@ -1,13 +1,33 @@
 dist: xenial   # required for Python >= 3.7
 language: python
-python:
-  - 3.6
-  - 3.7
+jobs:
+  include:
+    - name: "Python 3.6 on Linux"
+      python: 3.6
+    - name: "Python 3.7 on Linux"
+      python: 3.7
+    - name: "Python 3.8 on Linux"
+      python: 3.8
+    - name: "Python, developmental version, on Linux"
+      python: nightly
+    - 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
+      langage: 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
+  allow_failures:
+    - python: 3.8
+    - python: nightly
 git:
   depth: false
-before_install:
-  - pip install poetry~=0.12.17
+cache: pip
 install:
+  - pip install poetry~=0.12.17
   # we run `poetry version` here to appease poetry about '0.0.0-source'
   - poetry version
   - poetry install