ci: Start testing on macOS (#446)

Author: dbanty

.github/workflows/checks.yml

@@ -11,10 +11,12 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
     strategy:
       matrix:
         python: [ 3.6, 3.7, 3.8, 3.9 ]
+        os: [ ubuntu-latest, macos-latest ]
+    runs-on: ${{ matrix.os }}
+
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python