modify tox testing command

Author: chrissimpkins

.github/workflows/python-ci.yml

@@ -23,6 +23,6 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install tox tox-gh-actions
+          pip install tox
       - name: Test with tox
-        run: tox
+        run: tox -e py39