Add coverage output

.github/workflows/ci.yml

@@ -81,10 +81,13 @@ jobs:
           python -m pip install --upgrade pip
           python -m pip install '.[dev]'
           # Build the extension module in-place so pytest can find it
-          python3 setup.py build_ext --inplace
+          export CFLAGS="--coverage" python3 setup.py build_ext --inplace
       - name: Run tests
         run: |
           pytest -vs tests/test_cpython_interface.py
+      - name: Show coverage
+        run: |
+          gcovr --filter vcztools
 
   c_test:
     name: C tests