workflows: Run linter on oldest supported Python

This is useful since it will notice use of too new futures (or lack of
"from __future__ import annotations").

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

.github/workflows/_test.yml

@@ -16,6 +16,7 @@ jobs:
         include:
           # Run macOS tests on 3.9 (current OS X python) and latest,
           # Run Windows and "special" tests on latest Python version only
+          # Run linter on oldest supported Python
           - python-version: "3.9"
             os: macos-latest
             toxenv: py
@@ -34,7 +35,7 @@ jobs:
           - python-version: "3.13"
             os: ubuntu-latest
             toxenv: py-test-gpg-fails
-          - python-version: "3.13"
+          - python-version: "3.9"
             os: ubuntu-latest
             toxenv: lint