Try fixing GitHub Actions to use the correct Python

Author: mgorny

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
       - name: Install dependencies
         run: python -m pip install tox
       - name: Run tests
-        run: python -m tox -e py
+        run: python -m tox -e py3
   lint:
     name: Lint
     runs-on: ubuntu-latest