chore(ci): remove legacy Travis; lint uses .python-version

Travis config targeted Python 3.7–3.9 and pip; GHA is canonical.
Lint workflow reads the tracked python-version file for DRY.

Made-with: Cursor

Author: tommy-ca

.github/workflows/lint.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "3.12"
+          python-version-file: ".python-version"
 
       - name: Sync dev tools
         run: |