Add tests to GH Actions

.github/workflows/tests.yaml

@@ -0,0 +1,25 @@
+name: tests
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Set up Python 3.
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.12'
+
+    - name: Install dependencies
+      run: python -m pip install pytest pytest-cov
+
+    - name: Run tests
+      run: python -m pytest tests --cov=oscar_python