ci: add macos to matrix

Author: machow

.github/workflows/ci.yml

@@ -11,7 +11,11 @@ on:
 jobs:
   tests:
     name: "Tests"
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: ["macos-latest", "ubuntu-latest"]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2