Add code coverage

Use pytest-cov and codecov to collect information about coverage and automatically generate reports in codecov.

.github/workflows/python-app.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 pytest setuptools wheel build
+        pip install flake8 pytest pytest-cov setuptools wheel build
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
         pip install .
     - name: Build binary wheel and source tarball
@@ -38,6 +38,10 @@ jobs:
         flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
         # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
-    - name: Test with pytest
+    - name: Test with pytest and collect coverage
       run: |
-        pytest
+        pytest --cov --cov-report=xml
+    - name: Upload results to Codecov
+      uses: codecov/codecov-action@v4
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}