Fix tests by excluding broken version combo

Jupyterhub 0.9.6 used a deprecated import (sqlalchemy.interfaces) that was removed in Sqlalchemy 1.4. The python 3.5 environment only installs 1.3, but the python 3.9 environment now uses 1.4 and thus Jupyterhub fails to run. Excluding this test instead of adding a version pin since running such an old JHub in a very modern environment should be quite rare.

.github/workflows/test.yml

@@ -40,6 +40,9 @@ jobs:
           # JupyterHub 1.3.0 requires python 3.6+
           - JHUB_VER: "1.3.0"
             python-version: "3.5"
+          # JupyterHub 0.9.6 used a deprecated sqlalchemy feature removed in py3.9 environment
+          - JHUB_VER: "0.9.6"
+            python-version: "3.9"
         include:
           - JHUB_VER: "master"
             python-version: "3.9"