Update Codecov action

Author: itssimon

.github/workflows/tests.yaml

@@ -42,7 +42,7 @@ jobs:
       - name: Run tests and create coverage report
         run: poetry run make test-coverage
       - name: Upload coverage report to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
           token: ${{ secrets.CODECOV_TOKEN }}