Update Python version to 3.8 for json linter

Signed-off-by: Francis Guimond <francis@sensu.io>

Author: fguimond

.github/workflows/lint-json.yml

@@ -11,8 +11,8 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: 2.7
+          python-version: 3.8
 
       - name: Lint JSON files
         working-directory: ./tests
-        run: python json-validator.py --directory ../content --extension .md
+        run: python json-validator.py --directory ../content --extension .md