Update HTMLTidy.yml

Author: guwirth

.github/workflows/HTMLTidy.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.6'
+        python-version: '2.7'
     - name: Install xmllint
       run: |
          sudo apt-get update