Add docs requirements to lint workflow

.github/workflows/lint_test.yml

@@ -37,7 +37,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install -r requirements.txt -r requirements-tests.txt
+        python -m pip install -r requirements.txt -r requirements-tests.txt -r requirements-docs.txt
         python -m pip install pytype # Not in requirements as it doesn't work on Windows
 
     - name: Run black, pylint and pytype