Add Codecov upload step to CI workflow

Author: SaridakisStamatisChristos

.github/workflows/ci.yml

@@ -14,3 +14,7 @@ jobs:
           python-version: "3.11"
       - run: python -m pip install --upgrade pip pytest
       - run: python -m pytest -q
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}