fix: tox refusing to run env without py prefix

Tox 4.9 refused to run any env that is not explicitly declared.
Environment starting with "py" are exempt. We only want to run envs in
CI with a specific salt version and whatever Python version is present.
The specific Python versions are managed by GitHub actions.

Therefore, prefixing the called env with  "py-" should have Tox run that
env with any current Python 3 version and our specific salt version.

.github/workflows/test.yml

@@ -27,7 +27,7 @@ jobs:
         with:
           python-version: ${{ matrix.python }}
       - run: pdm install --dev --no-self --no-default
-      - run: pdm run tox -e salt${{ matrix.salt }} -- -vv
+      - run: pdm run tox -e py3-salt${{ matrix.salt }} -- -vv
 
   black:
     runs-on: ubuntu-22.04