Fix test workflow. Checkout with submodules.

Author: jg-rp

.github/workflows/tests.yaml

@@ -12,6 +12,8 @@ jobs:
         python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]
     steps:
       - uses: actions/checkout@v3
+        with:
+          submodules: true
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v4
         with: