add windows and mac builds to ci

Author: ssteinbach

.github/workflows/python-package.yml

@@ -12,9 +12,10 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
+        os: [ubuntu-latest, windows-latest, macos-11.0]
         python-version: [2.7, 3.7, 3.8]
 
     steps: