Run pip check.

.github/workflows/pythonapp.yml

@@ -22,6 +22,10 @@ jobs:
         mamba install -y nbmake pytest-xdist line_profiler  # add'l packages for notebook tests.
         mamba install --file requirements.txt --file requirements-extra.txt
         python -m pip install -e .
+    - shell: bash -el {0}
+      name: Run pip check
+      run: |
+        pip check
     - shell: bash -el {0}
       name: Lint with flake8 for pull requests
       if: github.event_name == 'pull_request'