Use coveragepy for generating coverage

.github/workflows/coverage.yaml

@@ -28,11 +28,12 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install pytest-cov codecov
+          pip install pytest-cov codecov coverage
           pip install .[test]
       - name: Test with pytest
         run: |
-          pytest --cov=catmux --cov-report=xml .
+          coverage run --source=src -m pytest && coverage report
+          coverage xml -o coverage.xml
       - name: Run example
         run: |
           catmux_create_session -d catmux/resources/example_session.yaml