test on MacOS + Windows

Author: rmorshea

.github/workflows/test.yml

@@ -7,10 +7,11 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
         python-version: [3.7, 3.8, 3.9]
+        os: [macos-latest, ubuntu-latest, windows-latest]
 
     steps:
       - uses: actions/checkout@v2