ensure all requirements are installed

Author: rkm

.github/workflows/ci.yml

@@ -11,6 +11,6 @@ jobs:
       - uses: actions/setup-python@v2
         with:
           python-version: 3.x
-      - run: pip install mkdocs-material
+      - run: pip install -r requirements.txt
       - run: git fetch -u origin gh-pages:gh-pages
       - run: mkdocs gh-deploy --force