add pytest-cases

Author: Matt Sokoloff

.github/workflows/python-package.yml

@@ -59,6 +59,7 @@ jobs:
     - name: Install package and test dependencies
       run: |
         pip install tox==3.18.1
+        pip install pytest-cases
 
     # TODO: replace tox.ini with what the Makefile does
     # to make sure local testing is