add command pip install requirements.txt in build step

.drone.yml

@@ -20,7 +20,7 @@ steps:
   - name: site
     path: /drone/src/docs
   commands:
-  - cd doc && mkdocs build
+  - cd doc && pip install -r requirements.txt && mkdocs build
 
 - name: deploy
   image: drillster/drone-rsync