CI: add doctestplus demo to CI

Author: ev-br

.github/workflows/pip.yml

@@ -86,3 +86,9 @@ jobs:
             for file in "${test_files[@]}"; do
               python -m pytest "${file}" --doctest-modules
             done
+
+      - name: Demo interoperability with doctestplus
+        run: |
+          python -m pip install pytest-doctestplus
+          python -m pytest scipy_doctest/tests/ -v -k'alt_checker'
+