Also test on MacOS and Windows

Author: pdecat

.github/workflows/build.yml

@@ -12,10 +12,11 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
         python-version: [3.5, 3.6, 3.7, 3.8, 3.9, 3.10-dev]
 
     steps: