Checkout submodules

Author: oschwald

.github/workflows/test.yml

@@ -20,7 +20,11 @@ jobs:
     runs-on: ${{ matrix.platform }}
 
     steps:
-      - uses: actions/checkout@v2
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          submodules: true
+
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v2
         with: