fix(ci): ensure six is installed

Author: foosel

.github/workflows/docs.yaml

@@ -74,7 +74,7 @@ jobs:
       - name: 🐍 Preinstall some dependencies
         if: github.event_name == 'push' || github.event_name == 'workflow_dispatch' || github.event.client_payload.ref == format('refs/{0}/{1}', matrix.source, matrix.version)
         run: |
-          pip install sphinx==${{ matrix.sphinx }} sphinxcontrib-mermaid==${{ matrix.mermaid }} Jinja2==${{ matrix.jinja }}
+          pip install sphinx==${{ matrix.sphinx }} sphinxcontrib-mermaid==${{ matrix.mermaid }} Jinja2==${{ matrix.jinja }} six
 
       - name: 🏗 Set up NodeJS
         if: github.event_name == 'push' || github.event_name == 'workflow_dispatch' || github.event.client_payload.ref == format('refs/{0}/{1}', matrix.source, matrix.version)