Add in sphinx-multiversion.

Signed-off-by: Chris Lalancette <[email protected]>

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
 
       - name: Install dependencies with pip
         run: |
-          pip install doc8 sphinx
+          pip install doc8 sphinx sphinx-multiversion
           pip install git+https://github.com/osrf/sphinx-tabs
 
       - run: make html