CI: Run lint on oldest supported Python version

* This was suggested as best practice by a pylint developer
* Seems better than CI randomly breaking when GitHub updates
  Python version (and pylint starts applying new rules that we
  can't follow because that would break old Python versions)

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

.github/workflows/_test.yml

@@ -14,10 +14,10 @@ jobs:
       - name: Checkout TUF
         uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4.1.0
 
-      - name: Set up Python 3.x
+      - name: Set up Python (oldest supported version)
         uses: actions/setup-python@65d7f2d534ac1bc67fcd62888c5f4f3d2cb2b236 # v4.7.1
         with:
-          python-version: 3.x
+          python-version: 3.8
           cache: 'pip'
           cache-dependency-path: 'requirements/*.txt'