Add support for Mac OSX and Windows testing on latest Python version

Author: Josh Howard

.github/workflows/ci.yml

@@ -4,16 +4,22 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        python-version: [
-          3.6,
-          3.7,
-          3.8,
-          3.9,
-        ]
+        os-version:
+          - ubuntu-latest
+        python-version:
+          - 3.6
+          - 3.7
+          - 3.8
+          - 3.9
+        include:
+          - python-version: 3.9
+            os-version: windows-latest
+          - python-version: 3.9
+            os-version: macos-latest
+    runs-on: ${{ matrix.os-version }}
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2