.github/workflows/pages.yml: add Python for parsing

Attempt to add Python and Poetry so we can run the schema parsing
scripts to generate the site content and the RDF.

.github/workflows/pages.yml

@@ -6,6 +6,20 @@ on:
       - main
 
 jobs:
+  parse:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Install Poetry
+        uses: abatilo/actions-poetry@v2.1.4
+          poetry-version: 1.1
+      - name: Poetry version
+        run: poetry --version
   pages:
     runs-on: ubuntu-latest
     steps: